So I've had significant growth down stairs since starting T. What I'm trying to figure out is when they say "you'd grow between x and Y on T" where does the measurement point start? Does it start at the pubic bone like what a meta would basically be or is it the visible section protruding out of the hood?

Typically it's measured from where it connects to your pubic bone to the tip. So, as you describe, the length you would have with meta/release surgeries.

From the pubic bone to the tip.