I need to know how much torque is produced for a 45.6 in/oz servo using a 20:1 gear reduction.

Simple answer is it's 45.6 x 20. More force over a shorter distance is the same as less force over a longer distance.

A more accurate answer is that the gearbox has losses so the calculation is 45.6x20x(efficiency of the gearbox) where Efficiency is, say 50% use 0.5.

Paul,
The output of the gear reduction should be:
45.6 * 20 = 912 In oz
or 912 / 12 = 72 ft oz
or 72 / 16 = 4.75 ft lb

Hope this helps
PS: I think your original units should be in oz of torque NOT in/oz???