Most problems seems to be with the maths, and some people even struggle to use the calculators.
The scariest thing is that you can get carried away with buying stuff and end up with 5 years supply of concentrates of which you'll only ever use 10%.
Of that 10%, half will go down the sink with nicotine in it.
So just be sensible - buy PG, VG and nicotine in litre bottles, start off with a few concentrates and make small test samples. Gradually build up a portfolio of stuff you really like, and when you've found some favs, buy in 100ml bottles.
I've eliminated PG from my shopping list by buying nicotine and concentrates in 100% PG and adding only VG to the mix so it comes out at about 70/30. It doesn't save anything, but it's one less ingredient to think about.