Hi Steve
Your testing my memory here but in my film days, I found the use of a Hartman Mask to be very accurate when used in conjunction with a matt foccusing screen and a right angle finder (Olympus OM-1n), I dont know whether or not you have a removeable foccusing screen or not, I doubt it though.
The other way that focus could be achieved with great accuracy was by using a knife edge focusser, you may have to Google this as it has been a long time since I went down that track
and I have forgotten most of it.
Cheers
JohnG