Don't rely on the length of the adjuster bolts; the arm that they bear on has a tendency to bend so no guarantee that they will have the same free length when the adjustment is correct..
A while ago I did some measuring involving levels, plumb bobs, giant verniers and assorted other measuring tools and found that the two sides of my swing arm from the pivot point to the forward end of the cutaway for the spindle were the same so I now adjust the spindle alignment by having the belt a bit slack, a bit of tension on the spindle nut to give some drag then gauging from the furthest forward part of the spindle cutaway to the leading edge of the adjuster plate using a caliper to compare the two sides and get them even, then carefully drawing the spindle backwards till the tension on the belt is right before fully tightening it up and double checking the gaps with the caliper. If you go too tight you have to get it too slack again before tightening, as otherwise there is backlash in the adjuster plates to take into account.
Of course there is nothing to say that the output shaft of the bevel box is actually parallel with the swing arm spindle or the wheel spindle...I still have more measuring to do.
