Sorry Julie...I'm still trying to figure where I should post/be/start, etc. BTW your Punkin and my Riley could be twins except Riley is missing his left eye.
No worries, Cherie. It's fine. We'll just keep it here now.
When you feed him late in the cycle, right before his next shot, his insulin is close to its weakest point.
When you give a shot, some of it builds up in the body to slow release and some of it is used right away. Typically that results in a "curve" after the shot, meaning that the blood sugar drops over the next few hours, maybe 4-6 hours, then as that insulin wears off his blood sugar will begin to rise again until his next shot begins to take effect.
Feeding him at +10 means that there isn't much insulin left available to deal with the carbs from the food he's just eaten. That can act like a brake on the remaining insulin, and cause his blood sugar to zoom up until the next shot takes effect.
How cats respond to carbs is very insulin, but if I were you, I'd try to not feed him in the second half of the cycle - after +5/+6 or so - and see if that doesn't improve his numbers at preshot. Most people feed when they give the shot, and then give some smaller meals between then and +5 or so.
It helps to take notes on what you're doing with the food, so that you can look at it after a week or so and decide if it is helping or not. Some people put an * in the cell of the spreadsheet where they fed.