Adjusting the rangefinder on an M, whether vertical or horizontal, is easy. Getting the right tool to do the horizontal adjustment is a little tricky, but if you have something like a butter knife or a small spreader, the rounded end works really well since it lets you angle the tool into the slot on the eccentric wheel. See what you might have lying around the house -- or go to a kitchen shop or gift shop that sells such items.

I realize that properly adjusting the M rangefinder requires checking the near and mid ranges also, but I think if infinity is right on then you should be good to go.