The numbers you got out of Watts selector is only if you want to let it run all the way up to the 30psi limit, which you don't really want to do.
You have more than 50 gallons of water total in the system, even if you were draining it from the very bottom of the boiler. The boiler has something like 5 gallons or more in it all by itself.
Using the Watts tool, set volume to 55 gal, both the pre-charge & autofill to 12psi, and pressure relief number to 20psi (the very high end of the pressure you'd ACTUALLY want to see if you're starting at 12psi when the system is stone-cold). Using 100F/200F for temperature range and the tool gives you a minimum expansion tank volume volume of 8.8/1.6 for volume/acceptance. If you dial the temperature range back 100F/180F it gives you 6.6/1.2, which gets you within 7.2/2.9 limits of the Flex2Pro HTX60.
The HTX30 is only good for 4.8/1.6 which is marginal at best. If the system was 12psi at a cold start of 65F and the high-limit set to 170F, the minimum sizes calculated by the Watts tool is 3.88/1.4 for a 30psi relief valve. Bumping just the temp to just 180F brings it to the hard limit on acceptance volume for the -HTX30. If the system actually has more water than 55gallons you're toast (as the symptoms seem to suggest!).
The HTX60 should keep it from spitting water in your operating range unless the true system volume exceeds 55 gallons by more than just a little bit.
Purging air from a fully drained multi-story system can be a bit of a pain. Note that you'll be starting with COLD 50-55F water- you may have to bleed more water out of some as the thing comes up to the operating temperature range to keep it down to ~12psi @ 100F boiler temp. Hopefully you'll find a bleeder valve (or valves) at the top floor of the system- open it up during the initial fill and fill until it stop gurgling & spitting air. You will likely need to run several 10s of gallons out of the bleeder before enough air is purged to get it to that point, and you're guaranteed to still have air elsewhere in the system, so this may be an iterative process. It may flow fine for awhile, then develop a bubble at the top of the system interfering with flow, which you can then bleed off with the bleeder valve until it's running clear with no fizz, readjust the pressure, repeat as-needed. (Be really careful if bleeding it while the system is hot.) Bleeder valves are often (but not always) located at a radiator, and many/most systems will have an automatic air trap/vent to aid in purging the last bits of air as the bubbles work their way to the top of the system while it's running once you have it purged well enough to get reliable flow.
The temperature gauge on the return water line is important for monitoring the operation of the system. It's OK if it's running cooler on start up, but if it spends most or all of it's burn time with return water below 130F it can damage the boiler with condensation on the heat exchanger plates or damage a masonry/terra-cotta flue with flue condensation. Natural gas exhaust condensation is mildly acidic, causing corrosion on iron boilers an breaking down the mortar holding the masonry together if it's chronically condensing. Most systems are set up with a 20-25F delta-T, so if the boiler's output is normally 160-170F, the return water temps will be reliably above 130F, except during start ups (which is fine.) If you swap in a condensing boiler you would then want to lower the temperatures as much as possible to reap the highest efficiency, which is why they use acid-resistant plastic exhaust venting rather than venting into masonry chimneys.
You have more than 50 gallons of water total in the system, even if you were draining it from the very bottom of the boiler. The boiler has something like 5 gallons or more in it all by itself.
Using the Watts tool, set volume to 55 gal, both the pre-charge & autofill to 12psi, and pressure relief number to 20psi (the very high end of the pressure you'd ACTUALLY want to see if you're starting at 12psi when the system is stone-cold). Using 100F/200F for temperature range and the tool gives you a minimum expansion tank volume volume of 8.8/1.6 for volume/acceptance. If you dial the temperature range back 100F/180F it gives you 6.6/1.2, which gets you within 7.2/2.9 limits of the Flex2Pro HTX60.
The HTX30 is only good for 4.8/1.6 which is marginal at best. If the system was 12psi at a cold start of 65F and the high-limit set to 170F, the minimum sizes calculated by the Watts tool is 3.88/1.4 for a 30psi relief valve. Bumping just the temp to just 180F brings it to the hard limit on acceptance volume for the -HTX30. If the system actually has more water than 55gallons you're toast (as the symptoms seem to suggest!).
The HTX60 should keep it from spitting water in your operating range unless the true system volume exceeds 55 gallons by more than just a little bit.
Purging air from a fully drained multi-story system can be a bit of a pain. Note that you'll be starting with COLD 50-55F water- you may have to bleed more water out of some as the thing comes up to the operating temperature range to keep it down to ~12psi @ 100F boiler temp. Hopefully you'll find a bleeder valve (or valves) at the top floor of the system- open it up during the initial fill and fill until it stop gurgling & spitting air. You will likely need to run several 10s of gallons out of the bleeder before enough air is purged to get it to that point, and you're guaranteed to still have air elsewhere in the system, so this may be an iterative process. It may flow fine for awhile, then develop a bubble at the top of the system interfering with flow, which you can then bleed off with the bleeder valve until it's running clear with no fizz, readjust the pressure, repeat as-needed. (Be really careful if bleeding it while the system is hot.) Bleeder valves are often (but not always) located at a radiator, and many/most systems will have an automatic air trap/vent to aid in purging the last bits of air as the bubbles work their way to the top of the system while it's running once you have it purged well enough to get reliable flow.
The temperature gauge on the return water line is important for monitoring the operation of the system. It's OK if it's running cooler on start up, but if it spends most or all of it's burn time with return water below 130F it can damage the boiler with condensation on the heat exchanger plates or damage a masonry/terra-cotta flue with flue condensation. Natural gas exhaust condensation is mildly acidic, causing corrosion on iron boilers an breaking down the mortar holding the masonry together if it's chronically condensing. Most systems are set up with a 20-25F delta-T, so if the boiler's output is normally 160-170F, the return water temps will be reliably above 130F, except during start ups (which is fine.) If you swap in a condensing boiler you would then want to lower the temperatures as much as possible to reap the highest efficiency, which is why they use acid-resistant plastic exhaust venting rather than venting into masonry chimneys.