I'm not the biggest of people and fairly sensitive to exercise...but for a gentle walk I reduce it by 30%, the nurse said to do this 1/2 hour before yo=u do the exercise ideally so that the change is actually happening when you are walking. Some people find they need to keep the temporary bolus going for a bit longer afterwards, I find that just 1/2 hour after works fine for me. It is a bit of playing but a 25% reduction should give you an idea and you can tweak it as you work out what works for you. I think I am a little extreme as if I do proper exercise like a good run or cross train then I have to drop my insulin by 70% so that it stays level, but that is literally for the 1/2 hour before and whilst I exercise if I leave it that low for too long after then my sugar rises quite quickly!