Yay! It's fun with math time ;-)
From Big Cat's site, the prop ester has a molecular weight of 74, and the testosterone is 288, so 74 into (288 + 74) is the percentage of weight of the ester, or 20% ester and 80% testosterone for prop.
Doing the same research for test enant, we find the enant ester molecular weight is 130. 130 into (288 + 130) is 31% ester, or 69% testosterone.
Your 500mg test enant is really 69% testosterone and 31% ester, so the effective testosterone dose is only 345mg (plus 155mg of ester).
To get a 345mg dose of test using prop, you would need to divide by 80%, since prop is 80% test and 20% ester. That would be 431mg.
So 500mg test enant should be equal to 431mg test prop.