Here's a different perspective. The infinity adjustment got off somehow on one of my M6s. Here's what I did. There is a rather largish slotted screw centered in the round cam follower, under the roof of the dark chamber. You can see it with the lens removed. I adjusted the infinity focus with a slight turn of that screw. Here's what I mean by slight. I move the screw, not by intending to turn it, but by applying a little pressure--just enough that it does turn, but very slightly. More of a nudge than a turning. Here is my procedure:
Remove lens. Apply slight pressure to the slot, being sure to remember which way I've turned it. Let's say I try clockwise first. Now replace lens. Check rangefinder. Better, or worse? If better, continue nudging in same direction until perfect. If perfect, then done. Worse? Nudge the opposite way next time--in this example, counter-clockwise. Better? Touch up with small tweaks until perfect. The pressure needs to be only in the rotational direction, not pressing down on the adjuster (remember, it's delicate).
I use, as a target, a 1000 foot television tower that is probably about seven miles away. If were on a trip, and didn't have that option, I would set it to the most distant suitable target available. If that is a quarter mile, I would consider it good enough in a pinch. Good enough for a 35mm f/1.4, or 50/2, or even a 90/2.8; though I would stop the latter down to f/4 just to be safe. A target over a half-mile, I would be more comfortable with. The moon is often cited as an infinity target, but I would not want to be sticking a screwdriver into my Leica by moonlight!
About that screwdriver. It is pretty hard, impossible really, to engage the adjuster slot with a regular screwdriver. Even the shortest one will be too long. But you can grind the tip into an angled shape like that of an Xacto blade, so that you can go in at an angle. Also grind the width until it fits the slot nicely. Now you have your own emergency infinity tool to have along on a trip. Or I suppose you could grind one of the blades on a Swiss Army Knife that already has too many blades. For airline travel, you can't carry on any knives, but maybe a nail file with a recontoured tip would be OK.
So. That has been my experience. I hope someone finds it helpful.
