Regardless of how you frame shots, whether by guesstimating based on the 50mm finder marks or by using an external top mount finder (some are better than others), the rangefinder accuracy of the R-D1 is marginal with that much lens, much more so than perhaps even a 28/1.9 wide open and close up.
Get either the flip-up Nikon magnifier or the excellent replacement eyepiece magnifier from MegaPerls, which also has a built-in diopter.
Regardless, I would not use a 90 Summicron on an Epson wide open at other than infinity. I have used a Tele-Elmarit and even with the MegaPerls magnifier the percentage of images in focus was not to my expectations.
If this is for a still life or landscape consider making a shot and then checking focus with the magnified review image.