As Revolt says, D=VT couldn't be used due to the amount of factors not being taken into account. For that equation to work, you would need to assume a constant velocity (or take an average velocity), and even then you aren't considering the initial acceleration, the effect of gravity and drag on the paintball.
If you split the projection into the vertical and horizontal components, you are kinda getting somewhere in that you can use a relevant SUVAT equation to calculate distance for each component, but you would still need to know the time the paintball is in the air.
Probably best to forget all that physics cr@p and do a number of experiments (if you have the space), firing paintballs at different angles and measuring the distance. Although it doesn't happen a lot in a game, a paintball fired upwards at a 45 degree angle is gonna travel a hellova lot further than one fired horizontally, hence why safety is always a big factor in something like this.
-Will