If you set the alarm at the max cold pressure, the alarm will go off soon after you get on the road, heating up your tires and increasing pressure. I've seen my truck's tires go more than 10 psi over max cold inflation towing in desert heat. I have never seen clear guidance of what is acceptable, but I think the assumption is that if you follow the cold inflation limits, you will be OK. 10 or 15 psi over max is probably a good point to start paying closer attention.
As for what pressure to run your tires, I agree with others that you should run at max labelled pressure, as ride comfort isn't an issue. I believe this recommendation is pretty much universal with ST tires. One exception would be if I were seeing tire wear issues related to over inflation (excessive wear at the center of the tread).
Tire Load and Pressure Index Tables give you weight capacity vs inflation pressure, but they won't help you set your TPMS alarms. Here is a chart for ST225/75R15
. I'm no tire expert, and can't vouch for the accuracy of this site, but the numbers matched exactly with the manufacturer load pressure index chart for my truck tires.