The dressing Medifast sells is 70 calories, 6g of fat and I think 5 carbs in the serving. So.. less than that should be fine. I'm sure the spray is ok, I bought one too but didn't use it yet.
This Wish-Bone salad spritzer looks great. Looks like one serving is ten sprays for 10 calories/1 g fat/1g carb. You can get info at wish-bone dot com.