I did not take prednisone, but another steroid in the form of a topical ear drop (which wasn't supposed to elevate my bg, but it did!). I can tell you that my bg was also high like yours and it took massive doses of insulin above what I normally use to bring it anywhere near my target range.
Please talk with your doctor or at least his nurse or someone who can give you professional advice. I am on a pump and did a increase of 50% temp basal up to an increase of 80% temp basal. In addition to that I changed my insulin to carb ratios in half - meaning if my I:C for a meal was normally 1 unit of insulin for every 10 carbs - I changed it to 1 unit of insulin to every 6 carbs.
Again you have to "play" with the I:C ratio until you find what works for you. Be cautious when changing your rates especially if this is your first time with prednisone. Oh......and don't forget to check with your doctor.