I use Humco brand vg from walmart. Pg from tractor supply. 50mg no flavor pg from extremevapors. Then I find a flavor I think I would like. Lets say rootbeer. I figure roughly 20 drops equals 5ml. So I make batches of 100 drops without nic. That way I'm not wasting nic juice and it makes the percentages easy to follow. Lets say 30% flavor, 30%vg, and 12mg nic.That's 30 drops of vg, 30 drops of my rootbeer and 40 drops of pg. Now taste it. If it's good(and this is my rootbeer all day vape) I add the nic to a bigger batch. 12 mg from 50mg juice is 24 drops. So instead of 40 drops of pg you would put in 24 drops of nic juice and 16 drops of pg. So the recipe would be 30 drops vg, 30 drops rootbeer, 24 drops 50mg nic juice and 16 drops pg.
I hope this helps. And as suggested make small 0 nic batches for testing.