Reserve is how much you can comfortably use before your safe floor — 100% when full, 0% at the floor (time to recharge).
It's built from a few numbers:
These are estimates. The real aged capacity is a best guess until it's measured over some deeper cycles.
Surge loads are a different story. A big motor starting — like the 3 hp water pump — can briefly sag the voltage down to the cutoff and trip a shutoff even when Reserve looks fine. That's about instantaneous power (and older batteries sag harder), not stored energy — Reserve tracks energy, not surge muscle.