I can't answer your baseline metabolism question, but everything I've ever read makes me believe 500 would be impossibly low (as in literal starvation). Here's a site that has some interesting information about it:
her "living thin" page is also good
Something else to consider: Are your measurements changing? If you are exercising 2-4 hours a day, you may be converting fat into muscle. The muscle tissue is more dense than fat so it will be smaller in dimensions than an equal weight of fat. If you are building muscle, the scale may read the same, but your measurements should shrink.