find exact center of tonearm bearing

The device arrived and i used it quickly and easily. it looks like a 90 degree prong that is about 3 inches long and a half an inch thick. a seperate piece of metal is attached above the prong dissecting the 90 degrees at 45 degrees. you simply place it on top of the bearing cap and mark the 45 degree line,spin the device clockwise 33 degrees and do it again, then a third time. if you have snuggled up the 2 90 degree prongs to the perimeter of the circular bearing cap all 3 lines will intersect at the exact middle. my bearing cap is brass so the ink marks wipe off.

If you use a set of dial calipers to measure the diameter of the bearing housing, then measure the spindle to front of housing instead -
The pivot to front of bearing housing distance should be :
Recommended pivot to spindle distance less ( 0.5 of diameter measured ) - assuming bearing is in centre of housing and that arm is level when you measure.
If you cant find the centre of the spindle then do the same - use dial calipers to measure the diameter. The centre is halfway. So you can calculate the distance from the edge of the spindle nearest the arm bearing to the front of the arm bearing housing.
The easy way is to do the calculations, then simply make a plastic ruler to the correct length - spindle edge to front of bearing housing.