Thanks everyone for your comments - one more question.
A number of people have suggested that the hardest thing is replacing the shims. However, it seems that the people who have had the hardest time with this, having to use trial and error to get the right result.
However, I have noticed a few posts that suggest it may be easier with the "fosucingscreens" products:
Originally posted by noelpolar Quote
I didn't need to adjust the shims.
That is surprising since my Canon screen is about 0.2mm thicker. The thickness of the shim will make no difference to what the AF and sensor sees, only to what you see through the viewfinder.
Focusingscreen.com includes plastic shims with their screens. They are quite flexible and difficult to use and inferior to the genuine (metal) Pentax shims
Read more at:
https://www.pentaxforums.com/forums/6-pentax-dslr-discussion/301663-changing-...#ixzz3it9ouDln
A thing to remember is that there are two clips/frames. The outer one holds the screen itself and a not so obvious inner one holds the spacer shim. Since the EeS screen is of a different thickness to the Pentax stock one you'll also have to replace the original metal Pentax shim hidden under the second frame with the supplied plastic one.
Read more at:
https://www.pentaxforums.com/forums/6-pentax-dslr-discussion/301663-changing-...#ixzz3it9yx1bE
Taking these comments together, I am assuming that there shouldn't be trial and error involved. Rather, the plastic shims provided should be the right size, to match the camera with the new screen.
As such, the problem is the fiddliness of the plastic shims, but not the size of them. As such, as long as I can get them in, there shouldn't be any trial and error. It should just be correctly set up for focusing.
Does that sound right?
Thanks