Nevermind, I figured out what I did, the equation should be:
vy = vy0 + gt
vy = 0 at apex, solve for vy0
vy0 = -gt
Substitute vy0 into standard physics distance equation:
y = y0 + vy0*t + 0.5gt^2
y - y0 = -gt^2 + 0.5gt^2
sqrt((y - y0) / 0.5g) = t
vy0 = -g * sqrt((y - y0) / 0.5g)
Ballistics works fine now.
Here is a capx to demonstrate if anybody is interested:
Working ballistics sample
Maybe it can help other people.
EDIT: P.S., Do the forums support unicode? It replaced my square root symbols (unicode symbol U+221A) with question marks.