I vaguely remember something about setting the maximium aperture of whichever lens you are using on that camera.
I'd recommend finding the owner's manual and reading it before you fiddle with anything.
As for zinc-air vs. mercury cells, the voltage difference is very roughly .05v. I use the z-a cells in two Nikkormats, a clip-on meter for old Pentaxes, and a Gossen LunaSix with no accuracy problems.