I don't belive there's a way to tell.
I shall firstly point out that I have not tried skytest myself, nor have I done feast, yet anhow
But the feast test is designed for it's only purpose of selecting ATC trainees, no other considerations have been made in the design. I personally looked at skytest and then thought it didn't matter, because the feast test will most likely be made to take along a percent or two too much rather than sort out someone with potential enough to be able to take on the training.
If there's someone around here who passed feast and used skytest they might be able to give you a clue, but I personally decided that either I "have it in me" or I don't. And there's no way of knowing if skytest is an accurate counterpart to feast or not, so overall my personal opinion is that it's better to focus on doing other stuff for preperation.
Feast is designed to be made without preperation, and yes it's always good to be as prepared as possible. But my personal opinon is that this is an exception to that rule.