to get it right without complications.
you take 50 ml of 36mg/ml nic liquid, by adding 50 ml of 0 nic liquid you have 100 ml of 18mg/ml liquid.
to avoid mistakes and drops in the nic level you have to prepare the 0 nic liquid BEFORE mixing it with the nicotine liquid. so you take for example 20 ml of VG, now add flavor for the amount of liquid you plan to have. lets say a 2% flavor. so you add 2 ml of flavor. now add to that 28 ml of PG and you'll have 50 ml of 0 nic mix flavored liquid.
from now is a simple thing. just mix the 50 ml of 36 mg/ml nicotine liquid with the 0 nic mix you made. you end up with 100 ml of 18 mg/ml liquid.
double the amount of flavor means you add flavor for the resulting 100 ml. not for the 50 ml that you are currently mixing.
you have no idea how cheap vaping can be till you learn to diy liquids. now i buy liquids by liters, prepare for the whole year for myself and also share with friends.