Hi Tonywinsc
As Tobes says, you would have to use a mirrored protractor for the reasons he states.
For the SME 309, it should be LofgrenA/Baerwald DIN.
But importantly, it should be a TWO point protractor, not an arc, as you will have to make adjustments at the arm base rather than the headshell.
As an aside, this actually is interesting because it raises the possibility of using the SME base for measuring the variation in cantilever/cartridge body alignment. as the alteration in overhang/mounting distance for a given effective length means a specific change in offset angle. It also offers a practical way to measure typical accuracies by comparing the difference in base position for repeated setups by different users without altering the cartridge position.
The accuracy that the stylus can be positioned independently on an arc can be measured, and the variation from that position in the alignments can give a number for the offset accuracy...
Anyway, back to work...