Whichever is cheaper - buying a 500ml of pg with nic, or a 250ml of vg. Say you're making 750ml of juice - you need 13500mg of nic total to make it 18mg(18*750, forget flavoring volume for now), so you either need 50mg 250ml VG or 25mg 500ml PG, either of which contains 12500mg total, and you...