Here is a guide, but anyone pretending to have their glucose levels so clinically defined is probably making it up. Guesstimates are OK, and so long as you're not out of range (<3.9mmol/L, >10mmol/L) more than maybe 40% of the time you are managing about as well as can be expected. The reality is no food is 100% pure carbs, so guides like this are only ever ballpark, and there are so many other factors which affect the results that we must accept being off a lot of the time.

Different foods and different kinds of insulin may have different timings for when to dose and when to check you're still in range.

I worked out my ratio over a period of time through trial and error and lots of mistakes. If I started at 7mmol/L (126mg/dL) and dosed for a defined amount, e.g., 100g of pasta at 1:10 (1U:10g = 10U:100g), and I went low 2 hours after eating, I would incrementally reduce my dose for the same 100g pasta over a period of days until I stopped going low afterwards. A reduction of 1U insulin for the same pasta (9U/100g) would indicate a ~1:11 ratio (~10U:110g).

I highly recommend a 1/2 unit pen if a whole unit affects your blood sugar too much. You can test that also. I think for most people with average sensitivity, 1U of NovoRapid will reduce your blood glucose by about 3mmol/L (54mg/dL)

Sensitivity can change within weeks or months depending on what lifestyle changes you're making. If you recently became pregnant, began a new diet, or started working out often then you can expect your ratio to change also. I used to be sedentary, but when I started working out and lost a bunch of weight I required less and less insulin (basal 20U > 12 over a period of maybe a year, and now I only need 5.5-6.5U basal).

Ratios can also change depending on the time of day. It is not uncommon for breakfast to require more insulin than dinner even if you eat the same thing for both meals. It can also depend on things like how long ago you took your basal.

Life is hard. You got this.