Couldn't you get some 6mg unflavored nic with 100% pg and just end up with double what you have at your 50/50 preference? I mean that would only be 300 mls of juice. I make mine 450ml at a time.
I know lots of people that vape 100% vg and they dilute the juice down with distilled water or PGA (Pure Grain Alcohol). I personally cannot vape it so I think the first suggestion would be your best bet.