Another way of crunching the numbers is:

(Tire size * pinion * pi) / (spur * internal ratio)

This requires division only once. It is also easy to pull out part of the formula and call it a constant to remove all need for division at the track.

(pinion * tire size) * (pi/(spur*internal ratio))

pinion and tire size are the only 2 things likeley to change so you can pre-calculate the right half.

Using the example numbers above you would just do

pinion * tire size * .0134

The easiest though is to just make a table, but real geeks do it by hand.