Overhang,
AHA! Progress.
It appears your Gyro was used with some arm that needed a shorter mounting distance than a Rega. Fortunately, the Gyro's armboad is adjustable. I don't know the details but there are one or more screws you loosen, after which the mounting plate spins. You'll note the arm mounting hole is offset, so spinning the mounting plate changes the arm's distance from the spindle.
Try to make your spindle-pivot dimension 219-220mm. That's the optimum for a Rega to achieve 2-point Baerwald alignment, which is widely considered the best scheme.
If you can do that, visit www.TurnTableBasics.com and buy their $20 mirrored alignment protractor. The TTB is probably the most accurate protractor short of a $150 WallyTractor. You'll be all set.
Doug
P.S. Don't bother trying to align a cartridge using your Rega protractor with the arm at 216-218mm. It won't be possible. That protractor assumes 223mm. If you don't want to buy the TTB protractor and just want to use the Rega one, you'll have to swivel the arm's pivot point all the way out to 223mm.