The side bar or wiki has something about mixing your first bottle where it walks you through the math. It's not too hard, but it takes a little time

Simple, basic arithmetic. There is literally nothing special about ejuice calculators if you know basic math.

You could do the math on your own if you wanted.

It's just basic math. Your finished product is 100 %. If your flavor profile is 15% then the remaining 85% is PG, VG, and nic. Numbers dependant on size of mix, desired ratio, and potency.

If you download the calculator that is linked in the sidebar, it comes with a bunch of spreadsheets which contains pretty much every formula that is used in those calculators.