I just buy 36mg PG from wizard labs, then buy equal amounts of 0mg VG, mix 50/50 and I have my 18mg base. I hate when things get overcomplicated. I even wish the calculator was a little more simplistic, as it always gets confusing with the PG/VG/NIC fields.
I personally don't care if the flavor offsets my PG by 5% or my nic level by 1-2%.. it's all semantics. I tweak a recipe til it's good enough to be an ADV, then save it in the calc.. easy peasy.