Divide the number of teeth on the driving (crank) pulley by the number of teeth on the driven (blower snout) pulley, like this...
Crank pulley = 45 teeth
Snout pulley = 39 teeth
45 / 39 = 1.15 or 115%
In the above example you are driving the blower at 115% of engine speed, so the blower is 15% over-driven. If you wanted to reduce boost you could swap the pullies, and then you would have...
Crank pulley = 39 teeth
Snout pulley = 45 teeth
39 / 45 = .87 or 87%
In this example you are driving the blower at 87% of engine speed, so you are 13% under-driven.