While I didn't solve the scale problem, I found a workaround.
The more I struggled with scales, the more I realised they were just not fit for purpose for almost anything beyond liking a forum post. For example.
- Scales don't work correctly with marking guides. For example rounding up instead of down causing a Fail (1) / Pass (2) scale on an assignment with a pass score of 2 incorrectly reporting a pass score if the learner achieved more than 50%. (Annoyingly, they don't do this behavior on a rubric where you need 100% to pass.)
- They don't integrate at all with the quiz module, meaning you cant use a scale in a grade book also containing a quiz
My workaround; ditch scales completely and use the Letters instead:
- In the course grade book, set up a letter scale to match your proposed scale. You can use whole words or short phrases instead of just letters. This is already more flexible than scales - you can set proper boundaries.
- I can already see me using this for a pass / merit / distinction type scale with boundaries of, for example 50% / 75% / 90%
- Create all your quizzes, assignments, etc as normal. If using advanced marking in assignments, you MUST NOT use a scale. Use a value instead. Simple direct grading seems to work fine with a scale.
- In the grade book, set up each graded module to use a letter Grade Display type (Or letter with percentage or value if you want to see both). This is under Show more for the Grade Item
- Do the same for the course total and also any category totals that use the scale.