I use pretty much the same process:
1) Measure the proper volume of nicotine base
2) Add flavoring/s by volume
3) Fill the rest of the bottle with premixed PG/VG 50:50.
If a person mixes 100 ml at a time and use 100 mg/ml base, the calculations are trivial:
- The ml of base needed is the same as the desired mg/ml solution - so if you want 24 mg/ml juice, you put 24 ml of base in the bottle.
- The amount of flavoring is equal to the percentage of flavoring desired. So if you want 3% of one flavor and 2% of a second, you add 3 ml of the first flavor and 2 ml of the second flavor to the bottle.
- Using the premixed PG/VG, fill the rest of the bottle with the PG/VG mixture.
Of course, this ignores the carriers for the nicotine base and flavors. I buy my nicotine base premixed 50:50 PG/VG. I only use a few percent total of flavorings, so ignore its contribution to the PG total.