Not sure if this will be useful for you super-cycling heroic types, but I found a good website that suggests how to adjust extra carbs/insulin reductions for exercise: http://www.excarbs.com/
Interesting basic suggestion is that you should aim to replace 1g of carbs for every kg you weigh per hour. (So a 70kg person exercising for 30 mins should take 35g extra carbs or compensate for that amount by reducing insulin). They get more technical than that but I’m looking at the easy options first! Haven’t tried it out yet of course…