Forget accurate if you want or need accurate you need a Nikon F or F2. A direct vision camera is not accurate. The round corners are the least of your problems.
Amen to that. I went in Leica world from nikon F3. In the beginning I was also frustrated with M6 that shows only 85% of negative when focused on infinity (with 50mm lens), then I get M3 that was better, but also not 100% precise.
Then I learned to let it go... be in peace with imperfection of rangefinder or shoot with 100% viewfinder SLR .