The formula I like best come's from Nophead's build manual for the Mendel90. The forumla is:

E_STEPS_PER_MM = ((# Steps per revolution * # teeth large gear)/(# teeth small gear * effective hobbed bolt diameter * pi))

For example, if I had a standard 200 step motor running at 1/16th microstepping, that's 3200 steps per revolution. A common Wade's gear extruder ratio is 39:11. A 8mm hobbed bolt has an effective diameter of around 6.75. Plugging in numbers, we get:

(3200 * 39) / (11 * 6.75 * 3.14159) = 535.289

535.289 would be your starting point and then you'd need to run your actual calibration on that. Typically you'd mark your filament at a known distance, command your printer to extrude a fixed amount less than that distance, then measure to see how much was actually extruded. Again, for instance, mark your filament at 110mm from the extruder. Command extrusion of 100mm. Measure between your mark and your extruder. Subtract that from your original starting distance to get how much was actually extruded. Say you actually extruded 103.45mm. You would then multiply your starting point of 525.289 by 100 / 103.45 to get 517.437, a little less since your extruder overextrudes slightly. If your extruder only extruded 98.6mm instead of 100, then your formula would be 535.289 * 100 / 98.6 = 542.889, a little more since it underextrudes slightly. Repeat the process and it should be dead on.