Salad dressing syns

The vast array of salad dressings is mind boggling, lots claiming fat free, low fat etc. and more often than not I end up buying one that has far too many syns in a tablespoon.
So, would it be reasonable to use the 20kcal = 1syn rule, and therefore if it were less that 20 kcals a tablespoon it would be free?

The 20 cal = 1 syn rule applies to salad dressings as there is no free food allowance.

The magazine says dressings must be less than 40 cals per 100ml to be free.

HTH.