I only buy 100 mg / ml, 100% PG nic base. Makes everything a lot easier. It's easy to dispense -- pg flows freely in an 18 AWG needle into the measuring syringe. Makes it easy to calculate -- I first add the % of VG I want in a mix and then everything else, including nic, is obviously not VG. So for example, say a 10 ml mix of 25/75 at 3 mg / ml nic with 3% flavor. 7.5 ml VG, 0.3 ml nic, 0.3 ml flavor, 1.9 ml PG. Done.