How to cut a piece of 2x3 wood crossways perfectly?
One of the most important things about making a stool is getting the legs the same length.  I would suggest you modify the miter box something like this:



Keep everything square when building the stop piece.  Then when you actually go to cut the legs, butt a square end to the stop, clamp it down, and cut.  You will get three or four legs precisely the same length.

Keep the stop piece.  You can move the stop around easily, and you will use it again.

Time spent making a jig or a stop is never wasted.