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candycan, that's not necessarily true, which further proves why brastrap is not a good indicator. It depends on how wide your bras strap is. The bottom of the bra in the image below isn't that far from the top and unless one is wearing the strap low down on the back, it may not be anywhere near midback territory:
MBL means just that, it's in the middle of your back which IMO lies halfway between base of neck and just below small of back. For some it may be close to BSB and for some close to WSL. But as a length check, each person's achievement is based on their own reach of these marks on themselves, not on another's positioning of the parts so it doesn't matter if the references differ from one to another. Thing is, at least those areas stay at the same place for each person:
Here's another of my charts: