I put 6 pinballs on the top glass by the speakers and let them roll down to the lockdown bar, timing them precisely. After a standard deviation calculation to ensure no outliers, I calculate the existing angle of level based on the mean roll time duration (and known distance, weight of pinballs, physics, etc.) and then adjust accordingly applying a known bias of the glass vs. playfield.
EDIT: If using anti-glare glass, make sure the appropriate constant of friction for the specific brand is applied.
