If you ask ten different doctors how much and how long to take prednisone, you'll get at least ten different answers, and likely eleven or twelve!
I had a situation in 2006 where I had a bad allergic reaction to flagyl - a really bad reaction - and three different docs I saw gave me three different schedules for how much pred to take and for how long. It turned out none of them were right!
The truth is there is no right or wrong answer because nobody knows ahead of time how quickly you'll respond to it, nor do they know how quickly you can taper without causing problems.
So the only advice I can give is that if you're really struggling on 40, drop down some. For you, 30 might be effective without making you totally miserable. Be a little flexible, and listen to your body some.
You don't want the pred to make you miserable, and if you need it at higher than 40 for more than a couple weeks, I'd say it's time to try another Crohn's med. The goal with Crohn's med treatments is to avoid prednisone if at all possible.