Hi Colin...jsp again. I spent the last 20 minutes here at work trying to derive your formula. Because of the complexity in calcuating your formula, I assumed that the equation would obtain an exact answer. But looking at your fomula again, this must be just an approximation. The units in your formula don't make any mathematical sense. Here is your equation again...
Distance of Movement = (sine of angle / length of shot in feet) x 15mm
You're solving for "distance of movement", which should have units of length. The other side of the equation must have the units of length as well, but the right side is unitless! The units for "Length of shot in feet" and 15mm cancel out, and all you have left are the units of "sine of angle", but that is unitless. Is there more to your formula that we're missing?
Oh...wait. Looking at your formula yet again, i'm thinking that 15 is just a constant of proportionality, and has nothing to do with an actual real dimension of 15mm. Okay, i think i understand now. Don't send this formula to a mathematician, cuz he'll have fits. A more mathematically sound way to express your formula is as follows:
Distance of Movement (mm) = [sine of angle / length of shot (feet)] x 15 (feet*mm)
Here, at least the units agree with each other.

Okay...now let me see if I can derive this 15 number