Those kind of rules are not very useful, since they tend to oversimplify things. For example the whole business of buying filters based largely on the gallon per hour rating of the pump in the filter makes no sense. It completely ignores factors like media volume, which are more important than pump throughput.
A watt per gallon rule for heaters is somewhat more useful, but there is no linear relationship. For smaller tanks you need more watts per gallon than for larger tanks. We recently had a thread about sensible heater size for a 10G, and most people seemed to agree that 50W would be in the right ballpark, but some also had good success using 25W heaters and 100W heaters on 10G tanks. For arguments sake, lets say for small tanks 5 watt per gallon is about right.
Anybody I know who runs large tanks will agree that for a 300G tank you don't need 1500W in heaters. My 240G with 90G sump runs perfectly fine with two 300W heaters, putting it in the 2 watts per gallon realm. That's because in large tanks the ratio of surface area to volume is smaller. They loose less heat, since heat is lost via the surface.
I agree that ambient temp in the room is important... as is circulation in the tank. 9 months out of the year, a 250W heater keeps my 125g plenty toasty. When the room gets cooler in January-March, I have another heater to take up the slack.
One of the 75g's is very well circulated, and is running a heater sized for a 55g.