How many trans could a trans trans trans if a trans could trans trans trans? That is hard to make work.
Anywho, I think figuring out the numbers is a bit of a lost cause for the moment. There are just so many factors at play. There are two big ones that stand out above the rest though, the more contentious issue being: what counts as trans? Is it explicitly those who transition or desire to transition from assigned sex? Is it anyone non conforming regardless of transition or identity? Do you base it on identity, action? Is it established by a standard, or entirely self identification? (Ie: If you transition into a woman, but don't believe yourself to be trans, at least not anymore, are you still counted?) Etc, etc, etc. Then even if you do establish that, the big question is then, how do you count a population that in large part actively seeks to be invisible?
Honestly, I feel as though the 1.5% numbers are on the high side, or perhaps suffer from a not clearly defined level of inclusiveness (ie: many people think the number represents transsexuals as a cliche of what it means to be trans, when it is more likely an estimate for people who are anywhere in the transgender umbrella).