I have done quite a lot of work with my low-carb meals recently! I find that generally, for a main-size lunchtime salad I need 1-2 units to cover it. If I have fruit or something with it I'll need 3. That is when I am at work, and using a lot of energy. I do find, that if I'm sitting around on my bum I might need more and, as has been said, if my sugar levels are out of range before I might tweak up or down a unit or 2.
However, I would say that it's what works best for you. You need to maybe eat a salad and say take one unit. If you're high later on, then next time you eat a salad take a unit or 2 more, depending on how high you were after the previous salad. But I quite often find that even if I am a little higher after an couple of hours, by my next meal I'm back within range because of the lack of carbs keeping the levels raised! Good luck!