Pre-packed salads with dressing sachets?

I'm sorry if this has been done before.

How would you usually calculate a shop bought salad box, with a sachet of dressing, that you don't use?

I hate most salad dressings and use balsamic vinegar and/or hot pepper sauce instead.

The salad I bought today had a ranch dressing salad. The box only had the calorie, fat, protein etc. values for the whole deal. I looked up the calorie 40g of ranch dressing and then took the difference of the overall calorie value and logged a % of the overall salad. My calories will be right but the other values will be wrong.

Is there an easier way?

Edited to add: The whole salad with dressing came to 240 cals and without, 70 cals.