Gentlemen, the OP asked what magification was required at ~500yds.
The answer is based upon 4x being considered sufficient/standard detail/discrimination for most all hunting targets (woodchucks to deer) at 100 yds.
Multiply that 4X by the relative yardage for the same detail/discrimination:
200 yds --> (200/100)*4X = 8X
300 yds --> (300/100)*4X = 12X
500 yds --> (500/100)*4X = 20X
This is the answer to the question.
Makes, models, variable-powers, field-of-view, ballistics, drop calculators, etc are interesting . . . but the answer at 500 yds is 20X