This article explains it better than I could:
https://optimisingnutrition.com/insulin-dosing-options-for-type-1-diabetes/
I could do a calculation of 3.5 units of insulin for 200 g (7 oz) protein plus a unit for 10 grams of vegetable carbs, but this doesn't work well as protein will take longer to digest so I will drop early and spike later. Anyway, my calculation gives me a rough 5 U for my keto diet meal plans.
It's easier to sugar surf, start with say 3-4-5 U and decide later if and when I need another 2 U or so based on my Libre rise. Sometimes it depends on how many carbs I'm eating (between 10 to 30), and how much exercise I've done, (on exercise heavy days I need to halve my fast-acting and reduce my night basal - my diabetes consultant recommended this), for what dose to start with. Sugar surfing allows for variability.