Hi Bigdog, you seem just a little confused
Your sum is correct as such but don't think you fully get it.
Normally when mixing a liquid, you would have a concentrate (one shot or otherwise) a majority of which in common use are PG based so that would need to go into your calculations.
Then you have your nicotine (this can be either PG or VG based) currently supplied at 72mg, depending on the strength of your liquid you won't be adding much of this or if higher then maybe up to a couple of ml's.
You would then have a bottle each of VG and PG which you would add to make the liquid to your desired consistency to vape.
So for an 80/20 liquid assuming your concentrate needs to be mixed at 20% the ingredients would be
20% Flavour concentrate/One Shot (PG based)
80% VG and VG based nicotine depending on strength.
If you really needed 80/20 then you wouldn't be able to use PG based nicotine, if the strength of concentrate needed is less than 20% say 10% then all you would need to do is add some PG liquid to cover this.
A useful tool that I use is E Juice me up
http://www.ejuice.breaktru.com/ you just add all the values in there that you want and it tells you on the right how much nicotine, flavour, PG and VG to add.