OK I think I got it, given my nick base at 40% PG the max VG I can have is 60%, found the 0 PG, VG check box by the flavorings.
Wallace
From the text above, I'm thinking your base is 60 mg nicotine, 100% PG?
Essentially, that works out to 100% PG for your nicotine base--60 mg is only 6% of the volume (a touch less), which would mean the PG is actually 94% (which is close enough to 100 to just call it that).
Depending on how much you dilute, which depends on what your target mg/ml of nicotine is, you can have a pretty widely varying mix.
Personally, I use 100% PG, 100 mg nicotine base, and end up with a 6 mg/ml, 50% VG liquid.