Working out really is a huge guessing game for each of us. It just really varies person to person? Trial and error?
I have found what I am used to doing routinely is going to be the most predictable.
I get on my exercise bike daily for at least 10 miles. Since I am retired I have the versatility to decide when I do. I usually save it for after I've eaten a full meal. Sometimes I try to take a little less insulin and get on when my numbers first start to rise. This also helps prevent some of the higher rises for me when I have judged wrong on timing or doses. It kind of stops it in it's tracks.
But when snorkeling which is much more of a workout, I have to start out at a higher number and decrease my basal rate on my pump. Usually I eat something within an hour afterwards. It depends on how much work the snorkeling has taken to how much of a drop I will have and if or how much I have to eat, it is much more of a variable that I have to adjust for at the time.