I prefer the glass bubble in the middle of the V if there is a flat spot... if not... I just zero out everything else, secure the fuse to the board and use a ruler/square to measure to a leading/trailing edge centerline points on the stab. I don't like all the excess weight of the Robart hanging off the rear as it can distort the readings IMO. Bottom line is.... whatever works for you and results in a plane carrying no corrective trim for a bad tail alignment.