I found i got down to needing 1 unit of meal time insulin for 3 or 4 gram carbs on low carb. Right now, on normal carb, i need 1 unit of insulin to between 13g carbs ( weekdays) and 20 (weekends). It changes week to week but thats where i am now. I get away with a low carb dinner as i am at work and really don't need hassle...
You may need different ratios for breakfast too. I tend to skip breakfast at weekends bit need about a unit or so to stop the rise you get at dawn.
I don't account for excercise, but eat sweets instead cos it helps with spontaneity...i mean, i could take less insulin, but what if it rains, or what if i want to keep going longer? Plus, i like sweets....
Incidentally, i find if i snack between meals, i take insulin for it but find that the snack food does not peak me like regular meals, even if the carbs are as high...it may be best not to snack too much at first until you have your ratios worked out