My nic comes in a 100% VG carrier. I did this for two reasons, one to make 100% VG juice for folks who are sensitive to PG (like myself), and two, it enhances long-term storage in the freezer.
To be honest, I've never used a calculator. It's really not necessary, although it helps to be a math whiz. Even if I did use a calc, it's still essential to know how to do it the long way to check yourself, or to continue if all the calculators suddenly fell out of the sky.
To determine nic, you need to work with the milligrams in their totality. You want to make 6ml of 24mg juice. You will need 144 mg of nic total (6mlx24mg=144). To get 144 mg of nic from your 36mg source, you would need 4ml (144mg/36mg/ml=4ml). So you would take 4ml of yoir source nic and add 2ml of VG to make 6ml of finished juice. Flavorings need to be subtracted from your VG. If you use say 10% peanut butter, 10% is .6ml. The 2ml of VG would be reduced by that .6ml for a total of 1.4ml VG, .6ml flavor, and 4ml nic source.