Wow that was confusing! The percentages used in describing boilies originally referred to the amount of protein in the mix. Nowadays it would more correctly refer to the amount of 'active' ingredients, ie protein, vitamins, minerals, etc. So a 50-50 mix would be 50% good stuff and 50% filler in the form of a flour of some sort and would generally be regarded as a short term bait. This way of describing baits has gone out of fashion a bit, but looking at my Little Black Book, Nutrabaits still do a mix called a 50-50 mix which they recommend as a value for money mix, particularly for fishing abroad.
Hope that helps