I realized last night that I was making the same mistake in a few places on the map that Koren n'Rhys pointed out on Thursday. Uggh, embarrassing to have problem with something so trivial, but I'll forge ahead and assume that other people might have the same problem.
It comes down whether you are counting the 10' corner square or not when you are giving dimensions.
To cheat and avoid the problem, I realized last night, that if we say there are torches in wall fixtures, we could say they are equally spaced. If there is one sconce per ten feet of wall, all we need to do is tell the new mapper how many sconces they see ahead. (You might also do this with decorative motifs carved/tiled in the floor: "You see two more murals carved in the floor before the corridor ends in a door").
Torch Radius Distances in Design
Another idea is that if you don't have any regular features like sconces, and you don't have light, you will need to use light source radius as a measurement. See a post here by Delta on torch illumination radius. So at pretty good light at 60 feet, we should probably take that into consideration when designing a dungeon easy to map. I mean, having doors or corridors at 30' would be easiest to describe: "You see the corridor turn to the right, just at the edge of your torch light." That seems pretty limiting though. Ideas?