When I worked in clothing retail, it was our understanding that the plus sizes picked up where the misses sizes stopped. So depending on the brand a 18W could be considered 20 or 22 if misses went past 18. I am sure things have changed in the years since I worked in clothing, and brands all have big differences, but I would think you can still look at it that way. How much smaller were the misses 18 compared to the 18W? I am finding that even in plus sizes, there are some brands I fit into the 18W and others I am still wearing 20W.
During times of universal deceit, telling the truth becomes a revolutionary act. -George Orwell