One of the best ways I've found to locate dead center is by using an inch/pound torque wrench...and get one which is the old style and not the "click" style (the long needle and scale up by the handle type is the most useful). I noticed Amazon has a couple for $21-$25. Using this (once the bearings are adjusted and the gears are relatively adjusted) you can easily see where things change. Mark that position with a line drawn on some masking tape on the shaft and box and then count the revolutions both ways to check. If you aren't in dead center you can adjust the box until you're blue and not accomplish anything. If you are rebuilding the box, it is dead easy. Find the center point of both, assemble it carefully and don't turn it, mark the center point with a Marks a Lot and there you have it.
