... I know that the bitcoin generation difficulty will stabilize at some point in the future, but at this point it would be useful to me to be able to calculate my ROI quickly and easily.
You're right, it would be useful, you just can't do that (anymore precisely than a weatherforecast for next year).
as mentioned, a PIII singlecore mobile-cpu running 150khash/sec may generate 50coins in 2hrs compared to a triple-core 1500khash/sec cruncher running 4 days without generating anything.
what would your forecast look like on those 2 machines and what would its users say/think, if the output isnt even close to what it tells them?