Calculating Missile Distance

[parkstan]

According Calculating Missile Distance the formula for missile distance is Velocity * Range / Screenlength. If you try two different inputs that must match based on the multiplication rules like 24 velocity and 12 range and 12 velocity and 24 range you can see that the missiles end up on different spots. The actual match of 24 velocity and 12 range is 12 velocity and 23 range. I have tried various inputs and ended with the following formula - Velocity * (Range - 1) / Screenlength.