I've been thinking about doing this as well. As long as 82/18 or 75/25 (due to different amounts of flavor in each recipe) is close enough then it'll be pretty easy with any juice calculator. I've been using
http://basecalc.solkku.com/ lately but it's still in it's infancy. Does what I need though and allows for cost calculations as well. A neat addition IMO.
I think of it in 100ml totals for easy math. First you need to know if your nic is PG or VG based (and what strength it is). For this example I'll use 100mg nic (VG base) for easy math.
To get 100ml total of 80vg/20pg 12mg base you would use ---- 12ml of nic (counts as VG), 68ml of VG and 20ml of PG.
If your 100mg nic is PG based instead of VG based, you would use 12ml of nic (counts as PG), 80ml of VG and 8 ml of PG.
Starting to make sense? If your nic is 36mg you'll use almost 3X as much as the examples I just listed. Shooting from the hip as I don't use this but if you had 36mg PG based nic, you can't make 12mg juice at 80VG. You'd be at 76.6 VG max because you'd need 33.3% nic (counting as PG) to get 12mg nic mixed base. Any PG based flavors would lower your VG ratio further.
Look at what total percentage of flavor you use and subtract that from the PG side of the base. If you usually use 10% flavor, make 90vg/10pg base so that you end up with 80/20 after flavoring.
A lot of rambling based on not knowing what nic you have, but post the calculations you come up with and we can make sure it looks right.