I also come from film era and crop viewfinders simply suck, they are composition finders at best.
Had the Katzeye with the K20D, going for this with the used K5 when it eventually gets here:
Focusing Screen
I think its the same S type everyone above is referring to though I chose the one with those grid lines because I can't seem to level things in the peep hole on my K20D the way I do the bay window on my Super Program so I figured a little help would be nice. I don't need the AF brackets as I never use AF or I wouldn't be getting the screen in the first place.
I found the microprism collar on my Katzeye never works, and the split prism makes the spot metering fiddly and I would rather like to use it more on the K5 so I ditched that too. The ground glass on the Katzeye actually works really well over stock by itself. Have to see what this stock K5 screen looks like but its almost certain I will replace it.
When you get the screen lay out something with a lot of print on it that is still really flat (newspaper can work if pressed flat) and pick a specific line of print to focus on at about a 45 degree angle, easy way to spot front or rear focus looking at the picture after, compared to what you focused on. Fast prime lenses that are reliable is best for this, try several lenses to check your answers. Just call Pentax parts dept (ahead of time so you have the shims already), and if by some miracle the one lady they seem to have doing everything actually manages to answer the phone tell her you want one of each shim for the K5. When I ordered them for the K20D they were labeled like A through H in their parts book she said, and were cheap enough where it wasn't worth the shipping to fiddle around so I just got all of them.