AIs (and 5ARIs for that matter) have extreme diminishing returns, it works out to something like 1/4th the dose works at 70% of full dose, half the dose works at 85% of a full dose, etc. For finasteride, 20% of the dose works almost the same as a full dose. These are well known pharmacokinetics that are shown in studies.
The whole "estradiol helps you grow" stuff is broscience though, it helps you hold water that is about it.
Of course, there are side effects of having E2 way too low, but that is hard to achieve on AIs unless the test dose is really low.