MassFX works fantastic for PCT.
As we all are hopefully aware, prohormone/steroids cycles decrease testosterone production, leading to lower test levels upon completion of the cycle. They also increase SHBG levels. So your total test levels get rocked, and your free test (the active form that builds muscle) gets hit even harder.
Taking MassFX will increase free test by inhibiting SHBG, which will allow you to "fake" being full go, while the rest of your PCT gets you there. If that makes sense to you.
I would recommend waiting maybe a week before starting, simply because the body is going to move pretty quickly on its own that week, and your total test is still oftentimes pretty low, to where it's not going to make much of a difference yet.