I generally approximate.. One thing I do too when I make a big salad for family...
Which includes olives, cheese, veggies and such.. I'll use the "Juices'' that the olives are in as part of the dressing. I'll use that in place of olive oil. I find it gives the right amount of oil and a different taste to the salad. then I add vinegar, salt and other spices if wanted.. Family loves it!
But olives are awesome and are pretty good for you!