The ACCA specs +2F as the 99% design temp for Syracuse, but OK, let's call that 0F. :-)
Forget the manual J for now- got a mid-winter gas bill, complete with the meter reading dates, and the average outdoor temp for the period? Short of an average temp, the exact billing dates and your zip code would let us look up the weather data for that period. That's a sub-2-minute calculation if I don't have to type it out, and it's a measurement, not an estimate. It'll put a hard stake in the ground for where the house is at right now. You didn't mention which size G124 you have- it comes in a number of different sizes, all run about 84-85% efficiency when tuned up, but if you look at the nameplate BTU-in and DOE BTU-out numbers it'll be good enough to use as a measuring instrument. With that we can reasonably size the boiler, but unless you have some ridiculous window/floor ratio you're looking at the smallest of any major vendors' mod-con lines, with a min-modulation output in the 15,000BTU/hr range, which is actually pretty easy to design around.
The Slant-Fin tool is going to deliver a number at least 25% higher than reality, but it'll be useful if you want get the room-by-room temperature balancing right.
Doing it with crusty old-schooler style rule of thumb WAG (but with more realistic rules of thumb) with 2500' of heated conditioned space with '90s vintage 2x6 & double panes, plus an insulated basement you're looking at about 35-37K of heat load at 0F, maybe 39K if you boost the basement temp to 72F up from where it's coasting at 65F or whatever. The smallest G124 has about 2x that amount of output for your, and would have short-cycling issues with stubby low-mass fin tube zones, and the bigger ones would have an even bigger issue. A zone with only 28' of fin tube puts out about 17K into the room at 180F, and the smallest G124 puts out about 4x that amount of heat.
Rather than adding more fin-tube to the existing zones, replacing sections of it with panel radiators gives a very real boost in comfort, especially near windows, since that brings the average radiant temperature up when you have say, a 38F window surface next to a 100F radiator surface. Fin-tube just heats the air, which is fine, but us humans are more comfortable at a lower air temp if the radiant temp is higher.
Any boiler will short cycle if you put just the minimum amount of fin tube for actually heating a basement zone with 140F water, but if you used flat panel radiators and 2-3x the minimum necessary you can probably have enough mass in that very-low load zone to keep it from short-cycling with a smallest-of-the line mod con. If you measure the amount of above-grade foundation wall area, and multiply by 5 BTU/hr that's about what your basement load is at 0F in BTU. Add another 3.5 BTU for every square foot of window. Assuming you have about 2' of exposure and a perimeter of 150' that's 300' x 5= 1500BTU/hr plus window losses, call it 2K. At at 180F it's only about feet of fin-tube, at 120F average water temp that's only 10 feet of fin tube, which is just too little mass or radiation for any boiler to deal with reasonably. But if you dropped in a Biasi B-24.71 it can deliver half the min-mod output of a tiny mod-con at 140F output, and enough thermal mass to keep it from going nuts short cycling. See: http://ecomfort.com/PDF_files/Biasi/...r_brochure.pdf
There is no such thing as balancing a basement heat load controlled by a first-floor thermostat- the change in heat load with temperature is just too different.
If you want to micro-tune the room by room balance on the upper floor zones, and combine the first floor zones into one (recommended) you can use the Buderus flat panels with the tweakable-tunable thermostatic mixers right on the radiator.
If you're going to do the work and spend the money, it's worth spending the money for an actual comfort upgrade, not just an efficiency upgrade. You may THINK you're comfortable with the fin-tube and ODR control, but it's nowhere near as nice as real radiation. (Radiant floors are even better, but an order of magnitude more expensive in a retrofit.)
If you have the headroom it's well worth putting down 1.5-2" of EPS under any new subfloor in the basement when you refinish it, which keeps the bottom side of the subflooring above the dew point of the room air in summer. With just a vapor retarder and no insulation you're asking for a mold problem at the ~45F Syracuse style deep subsoil temps. If you can't do the foam insulation for headroom reasonse, use only hard plastic or tiles, no laminates, no wood, and don't even THINK about using throw-rugs.