Instructions: But a number in either box and hit "Calculate." Note that all numbers are rounded, so they may be off slightly from actual in-game regeneration, though hopefully not enough that you'll notice.
Update, 2009-03-18: Holy crap, are people still using this? The formula used by this page is for pre-Wrath and is even more bogus with the 3.1 changes! As long as this message is still present, the numbers produced by this page are not to be trusted. I'll fix it sooner or later.