Another thing to take into account (as if Type1's don't have enough to think about!) is after excercise, your metabolism is higher, thus you may need more carbs after excercise to get your levels right (I do, but everyone is different)
Are you on multi daily injections (MDI's) or a pump? With my pump, I blood test after my Spin class and then input my carbs and have a tick box setting that takes off 20% off the total amount of insulin to allow for the post excercise 'burn' I get.
It is unfortunatly very much trial and error to get things right and even then its not 100% perfect, just to add to the 'fun'