There is nothing you can or should do to change the insulin. Once you inject it, it stays in the bloodstream for about 12 hours, until it's time for the next shot. You don't need to worry about spreading the insulin to match the food. It will all work out.
The feeder is a great idea. I would love to give some extra snacks to Callie, but with 4 cats and 2 dogs, sneaking in a snack is difficult.