Are you on a pump? It's easier to manage on a pump, but I guess you could figure it out for shots, too.
The way I manage cortisone shots is this (God bless my rheumy who figured this out): In the doctor's office, right before the shot, I set a temp basal for 125% of my regular basal. I then test every hour for the rest of the day, increasing the temp basal as necessary. Once my bg's stabilize (and they will if I get ahead of them), I keep the temp basal going for 2-3 days, then start tapering it off by the end of the third day. This usually works to keep my numbers reasonable.
I also give a bit extra for meal boluses, but I can't tell you any formula for that...it largely depends on what I'll be eating. With a high-carb meal, I usually just add 1 unit to what my pump recommends. With a low-carb meal, I'd probably just use what the pump suggests. And I do try to eat lower-carb for the three days after a shot.
If you're on MDI, I suppose you could figure out percentage-wise how much extra basal insulin to give yourself, and do the same thing I do with the meal boluses.
I've had numerous shots in the past several years, and I've been able to keep my numbers at least under 150, even after meals.
Although, some of this depends on how much steroid you're getting. My shots have usually been for bursitis or trigger finger, and as long as I get one shot at a time, this works. In September I got two shots simultaneously in both knees, and I didn't increase my basal enough...I was off the charts for days. I'll know better next time. But the main trick is to keep it from going that high in the first place because, as you probably know, once it goes up there from steroids, there's no getting it down until the steroids wear off.
Hope this helps.