As in, the prohormone would be dosed approx twice the mg of the target parent hormone. This is just broscience that more or less has worked well for most people, so take it with a grain of salt. There will always be diminishing returns and and exceptions, plus things to consider like how M1A and M14ADD don't actually convert very much to M1T and Dianabol, respectively - most believe they are quite anabolic on their own. Halo & its conversion to Turinabol also coming to mind.
That being said, it's a safe bet to assume you'll need more mg of a PH than of the direct steroid it's chasing. And it sounds like it's working well for Lad too so there you go.