In the extreme case, tape measures, protractors, etc.; for the most part: thumb, finger and scale.

Even at the edges, I'm not sure how good precise precise measurement is realistically. At 600m you have a 30% chance of hitting the target, at 601m, 15%; at 1600m you have a 5% chance of making a hit, at 1601m your never going to hit the target(never ever, not in a million tries, not a chance). How much realism does that add?

On the other hand, the, "the diagonal is the same as the orthogonal," assumption is nearly 50% off. This is equivalent to measuring 600m as being the same as ~850m. My fingers are better than that.

The square grid, or any grid, tends to constrain your design in unrealistic ways. The square page, and the need to fill it to the corners, is bad enough, let me tell you. Historically a ten foot grid means 10'x10' corridors. Have these people ever been inside a house? I'm supposed to believe I'm in a dank, claustrophobic cave that's roomier than my front hall? You know the corridors on the Enterprise(CT) were only eight feet wide? Even the 1.5m grid in the FASA Star Trek game were constraining. I miss that game.

The more I think about grids, the more I think, nononooo...

Why yes, I do enjoy my smilies...