Nhaler has a 60 mg liquid that comes in VG or PG. I use it constantly (must be mixed, do not use straight). Use the e-juice calculator to help if math is not your strong suit.
I make two bottles: 1) is 20 mg, 60+ 2 plain same size bottles VG (for me) makes 20 mg
2) 1 is 30 mg, 60 + 1 plain same size bottle makes 30mg
Then I work with these, other premixed, and flavorings and have a pretty close idea of what I end up with.
Hope this helps,