Hi Ame, I use a sliding back from Kapture Group. I was able to get accurate focusing using my old P45+ in Hassy V mount. When I upgraded to IQ180 in Mamiya mount, I also change the focusing screen and spacing. For my wide lens (40mm and 32mm) I have no problem using just a 4x loupe to get accurate focusing. However, the longer focal length, it seems to be off a bit even it looks in focus on the ground glass. I have not come around and get it adjusted and ended up using live view a lot. Now, I totally get used to the following workflow:
1. Initial focus and framing using the ground glass on the sliding back. I get to see "stitchable" area (approx. 6x9" size).
2. Slide the back over and turn on live view and view at 100% and fine tune focusing. Couple tricks here: (i) the lens must be in wide open position; (ii) use ND filter - I find that a 6 stops ND works perfect (even for a F4.0 lens) under bright day light and a 3 stops work pretty much the rest of the lighting situation. I use Lee filter holder and keep the adaptor ring on all the time. If you worry about exposing the front element, Lee has a lens cover that fit the size of the holder. So, no more screw on/off which is a huge time saver.
That's all. Hope this help.