What final ratio are you looking for? Bear in mind if you were using PG based Flavour and PG based nic and just VG for the rest, with a finished product that was 20% Flavour and 7.2mg nic your finished product would be about 30/70 PG/VG. So unless you require a higher VG content than that it might not be worth getting the VG based nic. I'm told that VG only requires refrigeration temperatures to make it stable, but lots of people do freeze their nic.
P.S. I used that example because I could do the maths in my head, but with a calculator much finer detail stuff is easy. I use ejuicemeup because I find it more intuitive than the others.