If you are buying 48mg, the highest % VG you can get is 10%
If your doubler is 30% VG and you mix:
a) the whole bottles of each (1 part e-liquid + 4 parts doubler), the 9.6mg mixture will be 26% VG.
b) the whole bottle of e-liquid with 3/4 of the bottle of doubler (1 part e-liquid + 3 parts doubler), the 12mg mixture will be 25% VG.
If your doubler is 20% VG and you mix:
a) the whole bottles of each (1 part e-liquid + 4 parts doubler), the 9.6mg mixture will be 18% VG.
b) the whole bottle of e-liquid with 3/4 of the bottle of doubler (1 part e-liquid + 3 parts doubler), the 12mg mixture will be 17.5% VG.
Of course, you don't have to mix it all at once. Tasty Vapor will send you a nice 15ml bottle to mix in. Just keep the ratios (1/4 e-liquid for 12mg; 1/5 e-liquid for 9.6mg) and you can figure out which blend is best for your before you commit the whole bottle.