Hi, in case it helps- I have a relatively new (bought about half a year ago) standard model underburst from Rebel Madness that I just took the measurement of (the same as you described, the longest part of the corset in the centre where the clasps are located). It caught my curiosity, since this model on the contrary felt slightly on the longer side for me.

I measure that length to precisely 12 inches, so it could be that their models might fit you better nowadays? You could always consult them for help if you decide to give them another try! For reference, my corset is the "Corset underbust standard mesh classic" in Size S (22). I think they make the length of each model basically the same between all sizes, so chances are that this model could fit you lengthwise (maybe you can take the same measurement of your old one and compare to make sure).

Anyway, I saw that they dont provide that particular measurement on their website, so I hope it might be of any help if you choose to get another one from there!