I'm glad you are feeling reassured. Its strange hey? Especially having to explain it to people "well, she's 5 and a half months, but because she was born 7 weeks early she is more like a 3 and half month old baby, so on some things she is right were she should be for her actual age, but others she's a bit behind, but she'll be all caught up by the time she is 2".
It sorta shuts some people up, I've noticed.
Betty's doc measures her at her actual age, but she takes her corrective age into consideration with everything she may be "behind" on.