I think your idea of measuring baby clothes is a good one. Manufacturers will vary, that's for sure, but any size you are given for "a baby" will only be someone's standard guess for size. Babies vary trememdously, and if you want to make something for a particular baby you'd be better off to use actual measurements from said baby. The baby clothes manufacturers have as good a guess as the next "expert".
My 15 month old granddaughter is wearing size 9 months in some things and 1 year in others. She is small, only weighed 17 1/2 pounds the other day. Many babies much younger than her wear bigger sizes than she does.
Good luck. Baby sizes are really hard. They should probably throw out the "month" sizings entirely and go by weight, height, and circumferences instead.