Assuming you and the target are either flying directly towards each other or directly away from each other then:

D = Distance

VT = Target Velocity (speed)

VY = Your Velocity

VSm = Slow Missile Velocity

VFm = Fast Missile Velocity

T = Time

FD = Fire Delay

D + VT * T = VSm * T = VFm * (T - FD) + VY * FD

Solving the right left equation for Time T:

T = D / (VSm - VT)

Now we solve the right equation for Fire_Delay FD:

VSm * T = VFm * T - VFm * FD + VY * FD

T * (VSm - VFm) = FD (VY - VFm)

FD = [ D / (VS - VT) ] * [(VSm - VFm) / (VY - VFm)]

And there you have it.

If your ship and the target aren't heading directly toward/away each other then a few trig terms appear, but same idea.