I do not know how the GM is measured, but I do know how the SP v3 is measured.
First, the endbell marking is WRONG. "Middle" is zero, "0" is -10deg, "+10" is +10deg.
Now, "zero" timing in this case is actually 30degrees mechanical timing, but this is a standard across all the motors as far as I know.
One way of finding zero would be to pull out the sensor cable while the motor is running on a zero-timing profile. If the motor runs slower in sensorless mode, the sensor timing is advanced.
For the SP, I would personally run it in the middle of the slot for timing-advance profiles, and at +10 for zero-timing profiles.