For nic, Final_Target_mg/Initial_mg is the % of the nic base to use in the final mix.
Example, you want 16 mg/ml and you have 36 mg/ml. 16/36 = 0.444, you need the final mix to have 44% nic base (4.4 ml per 10ml of final
eliquid).
(math check: Say I want 10 ml of 16mg/ml. 4.4 ml x 36 mg/ml = 158.4 mg of nic, which is consistent with wanting 16 mg/ml x 10 ml = 160 mg of nic. The rounding error is trivial for practical purposes.)
If your final goal is 70/30 pg/vg, that's 7 ml pg per 10 ml of eliquid, with the other 3 ml desired to be vg. PG nic base has used up 4.4ml of pg, so you want 7 - 4.4 = 2.6 ml of pg still to go.
Figure out your flavorings. If they are PG based count them against the 2.6 ml of remaining pg, else count them against the 3 ml of vg.
At the end you'll likely have a bit of residual, top it up with plain PG and VG.
Real world things can be more complex. I use primarily organic flavorings that have vg and some alcohol, though will switch to PG flavorings at times. I no longer bother calling things 70/30, 80/20, whatever. I'll call something 0PG, 0VG, or else just list the dominant ingredient (like 65PG). The 0PG and 0VG are precisely that -- mixes that have either no PG or no VG. The 65PG means that it is pretty close to 65% PG, with the remaining % mostly VG, but also a bit of alcohol in most cases.
I've downloaded multiple spreadsheets and other eliquid calculators, they're cool, just that I prefer to do math myself (keep a small calculator in the mixing area, if necessary.) Since I'm pretty .... about getting the nic right (I test my incoming nic base, to know what it is), I'm already going to do that first "how much nic?" calculation myself anyhow, and the rest is pretty simple.
