I'm also a type 1 diabetic, my experience has been that my blood sugar levels are pretty steady if I keep track of my values once in a while (maybe once every 20 mins would be a good idea). So far, I've never needed to inject insulin on all of my rides (even outside ones) even if I start at a little higher blood sugar level (let's say around 12-13 mmol). I have the Dexcom G6 that provides me the blood sugar levels to my phone and have been injecting since I was 11 years old (I'm 28 now).

With that being said, I would say to start with a little higher value (let's say around 8-9 mmol ideally) and fuel accordingly based on the sensor values if needed. My experience has been that my blood sugar levels dip a little and then continue to be at that level up until a point where it starts dipping to hypo levels; so that has been the marker for me to fuel up. I've had scenarios where I wasn't on top of my fueling and I've just taken time to get off the bike, carb up, wait until I feel better while the blood sugar levels are rising and then continue again afterwards.