When I started on insulin, my doctor gave me a starting ratio of 1 unit of insulin for 15 g of carbs
and I then adjusted from there. Adjustment was then done in
combination with close monitoring of blood sugars. Ie. I test blood sugar before eating, lets say it is 100. I then eat 45 g of carbs and inject 3 units of insulin before the meal. Test 2 hours after starting to eat and my blood sugar is 140 (my target is < 120 after eating). This tells me that the insulin was not enough.