As always, your diabetes will vary, but when I go out on the bike I drop my basal down to about 10-15% about half an hour before I go out (if – and it’s a big if – I remember). I will then knock a bit off my next bolus. Not very scientific but it seems to work.
During the Roman Rumble (when I cycled across Britain) I had a 10% basal during the day and put in 1-2 units of bolus for breakfast and lunch. It was as if I was cured!