I'm not sure where the 10% rule comes from but I can never find any research that says this. I don't disagree with it but has this rule ever been investigated as part of a research study?

I think the important thing to know is the following: "A gradual reduction in prednisone dosage gives your adrenal glands time to resume their usual function. The longer you are on prednisone, the longer it will take to taper off. The amount of time it takes to taper off prednisone depends on the disease being treated, the dose and duration of use, and other medical considerations. A full recovery can take a week, months (or years.)"
https://www.mayoclinic.org/prednisone-withdrawal/expert-answers/faq-20057923
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Sometimes people become steroid dependent and need prednisone for the rest of their life. This condition is independent of PMR/GCA. It just depends on whether or not their adrenals recover along with other factors.

When I was able to stay on 3 mg of prednisone an endocrinologist said I shouldn't attempt to go any lower than 3 mg. My cortisol level was too low to safely taper any lower than 3 mg so that was the dose I stayed on for months. Being able to stay on 3 mg was only possible because a different medication that didn't suppress my adrenal function kept my PMR symptoms under control.

Many months later after my cortisol level improved the endocrinologist said there was no need for me taper from 3 mg to zero because my cortisol level was "adequate." I wanted more assurances that my cortisol level was adequate. The endocrinolgist said I would feel it if my cortisol level was too low again and then I should take more prednisone with her guidance. There was no way to predict what would happen when I discontinued prednisone simply because I took prednisone for more than 12 years.

There was no magic formula that says one way to taper off prednisone is any better than another way to discontinue prednisone. The only thing the endocrinologist cautioned me about was the possibility of an adrenal crisis. In that case, I was instructed to take prednisone again for any reason if I felt the need. There was no way of knowing if an adrenal crisis would happen. That hinged on whether a very stressful event occurred. My daily cortisol level was deemed to be adequate for normal days.

All in all, there are just too many factors that come into play. I have witnessed people with GCA tapered off Prednisone very quickly because of steroid psychosis. The patient absolutely refused to take her Prednisone so it didn't really matter how slowly she went. She came out of the psychosis and was more reasonable later on. She agreed to take her prednisone again but a much lower dose was needed.