On a Rolleiflex with removable screens, no focus adjustment needed because the focusing surface, the bottom side of the screen, is always in the same plane.
On a Rolleiflex without removable screens, focus will most likely need to be adjusted. The screen's position is determined by the top surface, so the thickenss of the material determines the final position of the focusing surface.
The 'proper' way to adjust focus is to remove the lens shroud to access the viewing lens, loosen its mounting, and collimate the lens to the new focusing surface. This assumes thatoyur taking lens is already properly set to infinty. CRITICAL: it is important hat the taking lens and viewing lens are focusing at the same point.
Another perfectly fine way to adjust focus is to shim either the screen or the focusing hood depending on which way you need to move the focusing surface of the new screen. Measure the new and old screen with calipers. If the new screen is thinner (not likely), shim its top surface to make its thickness match the old screen. If the new screen is thicker, shim the outer edge of the focus hood- the surfaces where the four screws hold it in place.
If you don't have calipers, well.... maybe someone knows a way to do this.
Tape is a good shim material. To get a true measurement of atape's thickness: measure a flat plate like a piece of ground glass. Add a piece of tape, rub it down smooth, and now measure the thickness of the pplate plus tape. Subtract and that's what your tape adds. Measuring the thickness of the tape itself is not very accurate.
As is so often the case, Rick Olesen has a lot of info on this-
http://rick_oleson.tripod.com/index-175.html
He sells very high quality screens based on the old BrightScreen molds. And pre-cut to size.
It's a 7x7 screen, it won't even fit with out cutting, will it?
Right, it needs to be cut. And for fun and excitement, they are made of a soft plastic that can be marred and scratched.