Generally speaking, there are 2 schools of thought when it comes to forward lean- traditional and upright. Both work, but both depend on a combination of your biomechanical needs and what feels good to you.
Take 2 of our athletes Jossi Wells and Elias Ambühl- 2 super progressive freestyle athletes but they have completely different boot set ups. Jossi favors traditional forward lean, narrower last, stiffer flex while Elias favors a more upright forward lean, wider last, and slightly softer flex. Both work for freestyle, but each skier has different biomechanical needs and preferences.
FT allows you to change the forward lean by a couple of degrees. Not a ton, but it does affect your skiing. A boot-fitter can help guide you in the right direction by performing flexibility tests, but you can simply try out both and go with what feels more comfortable.