Mixing in a 1:1 ratio usually works well, so if you do this, you'll mix 12ml's bac water with 12000iu hCG. Then .12 or 12 units on the syringe will equal 120iu dose. When you mix 1:1 then finding your dose will always be the same whether you mixed a 5000iu vial, a 2000iu vial or 12000iu vial. I hope the following helps:
What you could do (just 1 way):
Add 2 ml's bac water to powdered hcg to dilute.
Draw back up the 2ml's of bac water that now contains hCG and put into the vial you will keep your hCG in.
Add an additional 10ml's bac water to the vial that now has your hCG.