RE: Someone talk me down...
It took me awhile to open this thread, because I figgered it was just someone that wanted to buy a Mathews, but didn't REALLY want to buy a Mathews. [8D]
It's likely that you would shoot a ring pin better than a solid pin. They're the cats a$$ for field archery (so good they were banned from bowhunter freestyle and bowhunter freestyle limited classes) and 3D. The eye just naturally likes concentric circles and automatically aligns them. The problems I saw with ring pins is they are fragile and they obscure too much of the animal on hunting shots - at least too much for my liking. Of course, on today's bows, rings on a multi-pin sight would be impossible. No way to get them close enough for 10 yard increments without overlapping.
Being up inside the housing of a pendulum sight, the ring should be well protected, so you could probably do okay with it.
I'd forget the math and select the size ring you need by trial and error. That's the best way.