A fitting question about the 40's......

So, gang.....

I have a really cool pattern for a six gored skirt that *says* it is my waist measurement.
BUT, when I put it on my dress dummy it was 3 inches too small. Yikes. Now did they figure it would be your waist measurement *minus* a few inches thanks to a foundation garment? Just wondering if anyone knew the answer to this as I will be working on many many vintage patterns in the near future.
Also wondering if it's okay to post these kinds of questions here, or if there is a more appropriate place to do so.
Hope you all had a fantabulous Labor Day!!

Miss Amelina