I've only reverse developed one premixed juice, but that was early on and I think my own creations are better now days.
I can taste a premixed and get a good idea of what flavorings are in it, but the most I've ever gotten from those are ideas for flavoring combinations I have not tried yet. Have yet to taste a premixed, in the last year, that I would bother to reproduce.
This leads me to a theory, if you've got enough experience to reverse develop a premixed, you are probably past the point where someone else's recipe holds any allure for ya'. That would seem to make a lab analysis the quickest and easiest way to go.
Maybe I'm biased because I started DIY because I wanted to make better tasting juice and not to save money? I see a number of folks post here wanting to mimic X's Z juice, but I've never had that desire. Also, it seems that the ones who want to make their own, with their own touch and combinations, are the new mixers that advance the furthest the quickest. (C Vap and Str8 come to mind as examples after I type that.)
I'll quit rambling now...