My experience has been that 500 is sufficiently far from 405 so as not to be able to predict this kind of thing with any kind of precision or accuracy at all. And if I did and was correct, it would be at least as much luck as skill, if not more.
I know that's not a satisfying answer, but it is the most correct one I can give.
As far as when you actually can hit 500? Well, depending how good you are at fives vs singles, your 5RM will likely be somewhere between 420 and 450, but without knowing more about you, it's hard to say more than that.