It's very non-critical, mysterious number. (uncheck your box so you have an identity!)
Lime juice works, citric acid works, lemon juice works, Apple cider vinegar works.
People have strong preferences, but they all work for me.
Also, when mixing the 5:1 drop ratio, it's not critical either. Anything over 5 drops of activator/per 1 drop Miracle-Mineral-Supplement seems to work.
Go slow....it'll be easy.
P.S. if you make a mistake mixing, just throw it out. It's cheap! Just don't make the mistake I did. Be sure to FLUSH it down the toliet or throw it outside. It'll make your drain smell like chlorine if you don't.