This is a great example. Somewhere along the way we realized/decided that there is no reason you can't place the opening Nemeth Code indicator at the end of the line in instructions or a cell-5 or cell-7 heading. I would put it after the word "equation." in the instructions. See the "solution" brf file, attached. **Note that you need to keep "12/36" and its switch indicators together on the same braille line. In an embedded expression, if the math and one or both switch indicators will fit on the line (within current margins), do so.
Back to your original question, a student will get used to seeing identifiers in both UEB and Nemeth Code. The transcriber's decision regarding where to switch depends on the surrounding material. If you tried to make all the identifiers in an exercise set "look" the same (in the same code), there would be much too much switching going on.
Hope this helps. Good luck with your project!