To give you some idea, prior to getting on the bike, I'd set a 0 TBR for an hour and a half, especially if I was planning on a long ride, and then set somewhere around 50% once that's done. The issue is always if you carry insulin into the ride so the initial kick off has too much IOB for the type of exercise.
I'd also consider setting up a "cycling profile" that I could switch to, which ran at much lower levels of insulin, so I was effectively TBR to zero from that, and when the TBR ended, I wouldn't have to set up additional TBRs while riding.
You do need to bear in mind that things like hill climbs and sprints flip you from aerobic to anaerobic exercise, so you'll likely see a glucose rise when you do those parts of the activity as well.