Going from memory, there are two different starter ring gears. 122-tooth and 149-tooth. Both have the same diameter.
The 122-tooth is thinner (front to back) than the 149-tooth gear. I have put a 149-tooth gear on a 122-tooth support. It will work with most starters but the SkyTec NL starter could have issues.
How many degrees per tooth? I would calculate that as 2.95=360 / 122 or 2.416=360 / 149.
I have a new unused 149-tooth ring gear in the hangar that I can go measure BUT I do not have a caliper that larger so will not be able to come up with an accurate measurement.
As to the math problem, just divide 360 (degrees in a full circle) by the number of teeth. So, 149 teeth: 360/149=2.416 deg or 122 teeth: 360/122=2.951 deg.
Somebody can check my math, but a 149-tooth gear has a diametral pitch of 12 (per some Lycoming docs I found), which means a diameter of 12.416", or a circumference of 39".