As I have posted in other threads, I gave up on watts years ago. All the current O-gauge power sources are limited by their current output, not the wattage; wattage is just a legality for UL and a marketing tool that lets manufacturers use big numbers! Just do it in amps, that's all the transformers and circuit breakers really care about!
Don't the HO guys rate their stuff in amps?
Actually, to continue down your path, measuring the primary current/power is a good way to see if a transformer is at or near overload, but the math involved in calculating output headroom in amps or watts is likely to be unpleasant to many. I think I'd just recommend not loading past 80% of rating as measured with your Kill-A-Watt.
