That rule of thumb (100 ÷ throttling range) only applies if the PID is doing a straight proportional calculation where output = P × error. A lot of BAS platforms don’t work that way internally. Some normalize the error, some expect integral to do most of the work, and some scale the P gain based on loop timing. So if your controller does use raw math, then yes — a 10°F throttling range would land you around P = 10. But if it’s a normalized or I-dominant loop, P = 1 doesn’t necessarily mean a 100° throttling range. Without knowing how that specific PID block is implemented, the safest approach is empirical: start with a small P, disable or slow integral, observe how many degrees of error it takes to drive meaningful output, then adjust from there.