You have less than 3/4 cuft of resin and only get 21K IF you set the salt dose at like 10-11 lbs. You will have a constant SFR of less than 6.75 gpm.
And 21K is the max K of capacity, so if you use that, your salt efficiency is going to be terrible, like 2000 grains/lb. And nothing should be run at max capacity IMO.
If you have a mechanical meter version of the 5600, the maximum gallons on the meter is 2100 BUT...
The 5600 in any version, requires a reserve capacity of one days total grains used so that if the meter gallons count down to 0, you run on the reserve capacity and have soft water until the next 2 AM when a regeneration can be done.
Some dealers will subtract like 20% of the capacity from the meter setting, I don't agree with that way of coming up with the reserve because the meter can 0 at say like 6 AM and 20% of a days use means you run out of capacity (soft water) before the next 2 AM.
You never answered me as to how many people there are in your household. How many?
How many bathrooms do you have?
What version of 5600 do you have?