I have a blog on the subject that goes into detail. Think of the recipe in parts. Flavor, PG, VG and Nicotine. If the recipe has 25% flavoring, as an example, that would be 25 drops. TFA flavorings are almost always in PG, so 25% or 25 drops is also 25% PG. If one wants a 70/30 VG/PG blend then we have 70 drops of VG and 30 drops of PG combined.
Flavoring is 25 drops (above example). Add 5 drops of PG for 30 drops, or 30% PG. If using 100 mg/ml VG nicotine, as I always do, and assuming 6 mg/ml of nicotine, then that's 6%, or 6 drops of VG Nicotine. Last, since we want a 70% VG mixture, we add 64 drops, or 64% to the 6 drops, or 6% nicotine, for 70 drops of VG and VG Nicotine. This example looks as following:
Flavoring - 25 drops, or 25%
PG - 5 drops, or 5%
VG - 64 drops, or 64%, and
VG Nicotine - 6 drops, or 6%
Added together equals 100 drops, or 100 percent. Do not worry about drop sizes, etc. Just use the same dropper sizes throughout. Viscosity is not an issue, flavor and taste is. Close is what we're seeking, not mathematical perfection. This is about speed and getting the recipe/formula roughed out.
If you are using nicotine, in a strength other than 100 mg/ml, then you have to convert that strength to percentage, and then to drops using a mixing calculator, if necessary.
Hopefully, this helps. Ask questions relative to this example, and I'll try to help in any way I can. Good luck!