I admit that if you couldn't do math correctly, I understand how you would have arrived at your incorrect conculsion ;)
The negative comes as you have....
(gamma)= 1/ SQRT( 1 - (v/c))
Where v is your velocity, c is lightspeed.
(delta)T=(gamma)(delta)T_proper
As you can see, placing a value of v greater than c gives a factor of
i (imaginary) which if you keep in there and then square out, gives a negative. It would also mean that the greater proportion of c you travelled above, the further backwards the precognition would occurr.
However as I said, this relies on generalisations of the Lorentx Theorem
The math that would follow through for a value of v greater than c goes back to principles and coms out in fucking insane algebra.