I think they don't tell you because they don't know!
Never mind too much info at once. I've had type 1 since 1987 and it was only in January this year when I did the Dafne course that I was told how to match my insulin to my food and how to work out correction doses. Most of the consultants at my hospital aren't Dafne trained so they never really mention mealtime insulin and just seem to concentrate on random meddlings with my background insulin. No good can come of that, in my experience, so I just stick with what I learnt on Dafne. I test before I eat anything, and if my blood sugar is high I take 1 extra unit per 3mmol reduction in blood sugar. So if I was at 20 I would take 5 extra units to try and get it down to 5ish. But obviously the ratio will be different in different people and there's a lot of trial and error involved. You probably have a higher insulin to carbs ratio when you're ill though. Hope you feel better soon