There is no accurate test to measure a cortisol level while you are still taking Prednisone. You need to be off Prednisone to get a semi accurate cortisol level.

Most doctors don't bother with obtaining a cortisol level because they are hard to interpret. There is a "normal range" for a cortisol level but there are too many factors to consider for interpreting what a normal level is.
----------------------
Artificial intelligence says it better than I can:

"While a general "normal range" for cortisol levels exists, interpreting what constitutes a normal level for an individual is complex due to various influencing factors. Cortisol levels naturally fluctuate throughout the day, peaking in the morning and decreasing later in the day. Individual differences, health conditions, medications, and even the time of day the test is taken can all impact cortisol levels, making a single measurement insufficient to determine a truly "normal" level. "