Many moons ago I ordered the Canon "S type" focusing screen from
Focusing Screen. Finally installed it today.
The metal shim in my camera was 0.5mm. The screen came with two plastic shims of 0.2mm, and one shim of 0.3mm. What I found most accurate was one 0.2mm shim. The shims are exactly the correct shape, and clip into the camera properly with the clip to the right of the main latch. Kudos to them for the attention to detail.
I used live view to calibrate, with a Pentax-A 50mm f/1.2 lens focusing on a tilted ruler on my desk. Did many iterations of the test, as each attempt at manual focus comes out a little different. I might wind up a bit more accurate with a 0.25mm shim, but it sure seems close enough.
The process is pretty easy with good hand-eye coordination and mechanical intuition. The two latches (screen and shim) work very nicely, better than in the Pentax LX in my opinion.
The screen really is pleasant to use with the 50/1.2 lens.
As Canon warns, the S screen gets dramatically dark at f/4.0. The Pentax-A 28-70/4 kit zoom is pretty dark. But f/3.5 is OK in my opinion.
The only f/4.0 lenses I have are that kit zoom, and Pentax-FA 20-35/4.0. The latter will be a bit of a bummer, although I've never really warmed up to zoom lenses. My A and FA primes are all f/3.5 or faster, so I think I'll be OK with this screen.
Obviously, the other choice is one of the Nikon screens, which don't have that cliff. But with so many interesting fast manual-focus lenses in my stable, I think I'll be happiest with this screen. I had a pretty poor hit focus "hit rate" with the Samyang 135/2.0 when I used it for a concert with the stock screen, I expect it to be more usable now.
The O-ME53 magnifier is still going to stay on the camera...