I'd like to think it was that logical, but I'm afraid it isn't. You could start by seeing how much a fixed amount of carbs raised your BG, but even that is hit and miss does to the GI, and individual's reaction to different foods.
When I started taking insulin, I'd already been carb counting for a long time, and started on a fairly low level ratio. That way you lessen hypo risk. Then I refined it in the light of experience.
Not much help eh?
Cheers
Barry