Re: barr and stroud rangefinder "zero"
I have both a Barr & Stroud and a Wild optical coincident range-finders, and I've found that it's very easy (and mandatory) to zero them from time to time. Just a few weeks back, I zeroed the Barr & Stroud by setting it on a tripod, carefully aligning the two images of Venus, and setting the range dial to infinity. The moon, the North Star, even a far-away con trail is handy for this. (The North Star is what I use to set my pocket transit to true North.)
Also handy are the power poles and the reflector posts along the highway. Count the number in a measured mile or two, to establish the distance from one to the next. Then set up next to one and zero on at least one, as far away as you can see it with all the intervening ones in sight (so you can count them without missing any, to get the total distance).
Changing zeros from season to season or even from week to week should never be a problem. Checking and resetting zeros is just too easy to be a problem or to disregard.