I just count however much I use and divide it among the portions. So, if I cook three chicken breasts in 1 TBS olive oil, I count 1/3 of a TBS oil if I eat one breast. Or if I prepare 4 servings of green beans and dress them with 1 TBS of olive oil and spices, I count 1/4 if I eat a serving.
If anything this is an overestimate, but I'd rather be over than under.
Jay