I ALWAYS do this with restaurant salads, because the restaurants I eat in rarely have low-fat dressings, or if they do they taste terrible. (Or, it's some kind of specialty salad where the dressing is a part of the composition.) So I order the regular dressing on the side and use this method, and it works like a charm.
Just yesterday I had an Asian salad (from Friday's it wasn't very good, no wonder I've started swearing off chain restaurants ...). I got the special dressing on the side, and it was EASILY a 1/2 cup! I wound up using about 2 Tbsp of it for the huge salad, and it was plenty.