I found this tip in a woodworking magazine and pounced on it. Basically, it's a guide to help me mark a guitar nut for string slotting. It's not the only way to do it, it's not the right or wrong way to do it, it's just the way I do it. Hope it helps somebody out there figure out string spacing.
The nut I'm demonstrating on is cut to width and ready to go on a 4-string neck. I know I freehanded the marks for the top and bottom strings in the video - I was showing the technique, not demonstrating my accuracy (or the lack thereof.) Normally I'd use a machinist's scale to figure where I want my top and bottom strings and mark the top and bottom that way. The marking gauge is to help locate the center string slots - nothing more.
If you already know what string spacing you want to use, you can just find a spot on the board where the lines are that far apart and mark the nut there.
I know that it looks like the marks I put on the nut are way off, and they probably are. I was trying to balance a camera, talk, and actually mark a nut at the same time - never a wise thing for me to do more than two things at once...