> a) what heuristic are you using to determine that power consumption will be 25% of the power supply sizing, and

I'm not the OP, but it's probably about right. Power supplies are cheap and there are variances in manufacturing. Typically power supplies for all consumer electronics are rated for double peak load. And assuming that standard load is half of peak load is probably on the high end - standard load for the processor(s) is probably more like 1/10 peak load, but there could be other components on the board whose electrical load is not workload dependent.