Suboptimal. The "NOT" might be covered by something, and then what remains is "THIS LEG". I would have written "NO NO NO" on one leg, and "THIS THIS THIS" on the other.

In aviation, standard phraseology is generally carefully designed such that (mosts) subsets of a phrase are distinct from the opposite phrase. For example, when ATC warns of traffic, you reply either "traffic in sight" or "negative contact". When ATC hears only half of either, they still know what you meant.

I know people who have been instructed to do this by medical staff prior to surgery. I have personally helped someone mark a mole they couldn't reach on their back.

Can confirm - had to point to the area that was supposed to be operated while laying in pre-op (or whatever it's called) room, just before being knocked out... Was quite scary ("you should know what you're doing!"), but quite sensible on a second thought.