I thought this was interesting enough to do an experiment. I have a Sigma 600mm mirror lens which is f8.
I popped it on a tripod and put an F3 on it and it worked perfectly, well, perfectly enough as it always has. My F3 bodies run the E focusing screen, which is the same as the B focusing screen, except there are vertical and horizontal lines.
I then popped a K focusing screen in and I got blackout (partial in one half of the split screen), so there is it seems an issue with the speed of your mirror lens and the K focusing screen, which like mine, is not very well suited to slower lenses. In this case, after market mirror lenses.
In literature that I have, the K screen only gives acceptable, not excellent results with their reflex telephoto lenses, which are: 500 f/8, 1000 f/11 and 2000 f/11. The B and/or E focusing screens are the only focusing screens for the F3 that get an Excellent rating for these reflex telephoto lenses.
I think you'll need to find either the E or B focusing screen for your F3 body, to be completely successful with your reflex telephoto.
Mick.