Calculator

Introduce the flavour concentration as a % (on each flavour bottle you'll find the suggested concentration), how many ml your flavour is (usually 10 or 30), choose how much nic you'd like (in mg/ml; if you recall the value as a %, multiply by 10; e.g. 2% nic = 20mg/ml), then choose the ratio VG/PG (usually 50/50 for mouth-to-lung, or 70/30 for direct-to-lung). 

Some common values of flavour concentration: shortfill 20+40, 33.3%;  shortfill 10+10, 50%.