I'm collimating my TPO-branded GSO 8" Ritchey-Chretien for the first time, and the first thing I want to do is get the focal length correct by adjusting the spacing between the mirrors and using plate solving to tell me the image scale / focal length. But there doesn't seem to be a good answer to what the focal length should be. There is about an even split between 1600 mm and 1625 mm out there, which seems to hinge on whether the aperture is 200 mm or 8" (203.2 mm). If you assume that the f/8 spec is exact that will give you the two focal lengths of 1600 and 1625 mm, respectively.
So I went and measured my primary mirror, to try to tip the scale one way or another. I used a U-shaped piece of cardboard with two marks exactly 200 mm apart, reached in and held this up against the mirror. I could get a pretty accurate reading this way. And guess what? The active, reflective portion of my primary mirror is about 3 mm LESS than 200.
I now have three different mirror apertures and no consensus on what the focal length should be. I don't even know if f/8 is exact or rounded. I've inquired to Guan Sheng Optical but I don't hold much hope for a response. The focal length of an R-C scope is supposed to be set pretty carefully to avoid aberrations, so what's a poor astronomer supposed to do?