I improvised a scribing tool out of a 1-2-3 block, an eye screw, and a pencil yesterday. It's not ground breaking, but it's a nice piece of frugality making use of something I've recommended you buy back in this earlier post. Make the tool or not; notice the method for stopping your chairs from rocking shown in the final photos.
The little block of wood is there because the holes in the 1-2-3 block aren't large enough to let the head of the eye screw pass through. If they were, the jig would be even simpler.
In practice, this worked just fantastic. I know the black chair legs don't look like you could see a pencil mark on them, but actually, the pencil shows up as bright, shiny silver on the black wood.
The method: on a large-enough, known flat surface that is level (check with a level first and adjust as needed), shim under the feet of the chair until it is all level. Then find the thickest shim and set the point of the pencil to that height. Trace around the other legs at the same height, cut exactly on your pencil lines, and your chair won't rock any more.