Undo the screws on the servo housing. Remove the gears on the left and right hand side so you just have the middle (bottom) gear left.
You will need to dremmel out the bottom of the main gear that fits over the pot as well as the ridge that acts as the end stop (I recommend you dremmel that as well rather than trying to snip it as first time round my snippers also broke the gear wheel in half). Having dremmeled the main gear then put it back in place and make sure it rotates 360 degrees without snagging. If all's ok then take it off again to expose the pot.
Plug the servo into the $50 circuit and download the 'hold the servo'.hex file. This will then pump out a 1.5 ms pulse to your servo which means 'Move to the center'. You should find that if you move the pot one way then the gear spins one way, turn the pot the other way and the gear spins the other way. If not - then something is wrong. So all you have to do is to move the pot to a central location where the gear stops spinning. The servo is then centererd and just needs a blob of glue on it to make sure that it doesnt move off center due to vibration etc.