You are exactly right, Rini and that's why we need to taper. The physiological requirement for cortisol is usually provided by 20mg/day of hydrocortisone, which would be equal to about 5mg pred. But the actual requirement, as you said, can vary. When you've been on pred for a long time, your adrenal gland basically shuts down and has to have time to wake up and start reproducing when pred is removed. If you quit suddenly or too quickly, then you risk an adrenal crisis.
I had an adrenal crisis when on a cruise in Tahiti several months ago and had to be air-evacted back to the main island hospital. The adrenal crisis was caused by different reasons, but the result was the same. I always carry injectable cortisone with me, otherwise, the outcome might have been different.
Adrenal crisis - Wikipedia, the free encyclopedia