Expert:
Has both PG and VG bases. Has nicotine in both PG and VG bases. Has flavorings in both PG and VG bases. Has the knowledge and experience to minipulate the ingredients (with their different bases) to achieve their desired final ratio (PG/VG).
Novice:
Has one base (maybe both). Has one bottle of nicotine (normaly in PG base). Uses whatever flavorings they can get when they order from somewhere (normaly in PG based). Uses the ingredients they have on hand to make mixes in whatever final ratio gets them close.
Having a floating "final ratio" and taking out the fixed "target ratio" will work for both.
An expert will be able to look at it as they go and make adjustments so this final ratio is right where they want it to be. They already know what ratio they want to achive.
The novice can insert what ingrediants they have on hand and let the calculator tell them what the final ratio will be.
This should eliminate the negitive numbers comming up and confusing the novice.
The only "target raitos" should be the nic. and total volume. These should never change as you calculate the mix. The final ratio can be flexable and should be allowed to change.
Now "in my mind" the perfect calculator would allow me to enter what volume I wish to mix (3ml, 6ml, 10ml, 15ml ...), the end nicotine ratio I wish to achieve (both volume and nic ratio would be constants), the bases I'm working with (100% vg base (or 100% PG, or 50/50 or whatever)),base nic. (PG or VG), base flavoring (PG or VG), water (also considered PG by vertue of it's vicosity).
The calculator would take all this into consideration as I enter my persentages. It would recognise that, if I'm using a 100% VG base, I am not using a PG base , that I'm only adding PG by vertue of the ingerdients and adjust the VG level as the values are entered.
If I use a different base (say 50/50) the calculator would recognise that I have some PG as a base and adjust that level acordingly).
As long as I have PG in my base liquid, and are adding PG based ingredients, it should take the same amount away from my PG base ratio. Once the base ratio hits 0 that calculator should recognise this and start taking volume from the VG side because you can't have more then 100% of your target volume. Even though your total "ratio" will float, your total "volume" should remain a constant.
For the most part, in recipes, many of the ingredients can't (or won't) change. The only flexable percentages are your bace liquids (PG or VG). These are what you have to take away from in order to allow the rest of the ingrediants to be added.