"At what point should you be measuring cortisol levels. And what is an acceptable level"

Testing for a low cortisol level isn't so easy to do. My endocrinologist said I needed to be "stabilized" on 3 mg of prednisone or less. I took that to mean I needed to be able to stay on 3 mg of prednisone without having a PMR flare long enough for my body to be "adjusted" to 3 mg of prednisone.

A cortisol level is hard to measure because of how prednisone interacts with the production of cortisol by the adrenals. The way I understand it --- your cortisol level will be low on higher doses of prednisone. That is because prednisone replaces the cortisol that our adrenals should be producing.

Supposedly 3 mg of prednisone is low enough to allow the adrenals to sense a low cortisol level. Ideally, the adrenals should react instantly but that doesn't happen when we take prednisone. It can take a long time for the adrenal to react. This is referred to as chronic adrenal suppression as a side effect from prednisone use.

CHRONIC ADRENAL SUPPRESSION
The goal in treating chronic adrenal suppression is to replace Prednisone in a fashion that will permit a reasonable quality of life and at the same time encourage recovery of normal hypothalamic-pituitary-adrenal function which is also known as the HPA axis.
https://my.clevelandclinic.org/health/body/hypothalamic-pituitary-adrenal-hpa-axis
This is a very complex mechanism and probably explains why we have PMR flares when we are stressed. Taking manufactured forms of glucocorticoids (corticosteroids) for rheumatology conditions may increase your risk of HPA axis suppression.

The best way to get this mechanism functioning again is to taper prednisone slowly so as to allow the adrenals time to start producing cortisol again.

Sorry ... according to my endocrinologist, there is no prednisone tapering scheme that works well enough to say one way works any better than another way.

Measuring my cortisol level wasn't that reliable either. My morning 8 a.m cortisol level was measured when I was able to stay on 3 mg of prednisone. My cortisol level was low as expected because I was on Prednisone for a very long time.