That schedule is borderline. However, since it is your first time on prednisone, it probably will work fine. If you start feeling unwell (low blood pressure, sudden weakness, etc.), you must have prednisone pills on standby, even 5 mg will be enough to make you feel better… The prednisone takes time (4-6 hours) to produce short term effect

From my experience, prednisone will work wonders first time, but whether the effect will be for long time, that is another question. You should evaluate yourself about 14 days after the last 20 mg dose.

Also, since the weaning is rather extreme, be prepared for a few days (day 2 .. 7) where you will really feel not well, and it will be difficult to distinguish if it the lack of cortisteroids or myasthenia which produces that condition