Also look at Diabetes.com a US website with a very good section on exercise, carb replacement & insulin reduction. It's in the diabetes management section of the website, they call it ExCarbs. This has helped me work out how much carb I use during exercise & taking into account insulin on board, starting BG, exercise duration it helps me calculate how much to reduce my insulin dose prior to exercise & how much carb I also need before/during exercise.
I find that I cannot exercise on insulin reduction alone, I always need additional carbs.
You need to know, or work out, how much effect 1u of insulin has on your BG & how much effect 10g carb has (if you know one, the website tells you how to work out the other). For example, I swim for 45 mins, my body needs about 40g carb for the swim. 1u drops my BG by around 2.5-3mmol depending on the time of day & I take around 1u for 10-11g carb. So if I start with a BG of 8, I can afford to drop 3mmol the equivalent of 10g carb, I then take 20g of carb before I swim & another 10 half way through, all of which adds up to the 40g (30g in actual carb & 10g which is already swilling around in my system).
I usually swim before dinner, so don't reduce my insulin dose as it would leave me high all afternoon.
However, if I exercise weekend mornings (cross training) I also calculate how much insulin to drop off my breakfast dise, probably 2 units, which means that I need 20g less carb during exercise to prevent the crash.
It does take a bit of getting your head around it as there's so many variables. You just need to learn how your body reacts to exercise & have fast acting carbs on hand just in case you crash.
Remember, that you will probably need to reduce insulin after exercise for up to 48 hours, depending upon how intense the session was, I am on a pump & run a basal rate of 80% normal from 4 hours after exercise until 24 hours afterwards to prevent hypos. Speak to your HCP about whether you should reduce basal, bolus or both, as it may be different for MDI.