When I have a long expression inside a figure - perhaps(2x-12)degree sign, my practice is to write a TN saying "x stands for (2x-12) degree sign."
My reading of the new keying rule is that I can no longer do this. I must use either a numeral (such as 1) or a two letter combination, such as tw) and put it in a key format, instead of just a plain TN.
I believe that the new keying rule is clarifying what must be used [u]if[/u] you are going to use a key. There are other options besides using a key. I assume that you are creating a tactile graphic when you say that the expression is inside the figure. You could check further with the tactile graphics moderator regarding label placement. Include an image of the print page if possible.