the less bac water you mix it with, the less volume you will have to inject to get the prescribed IUs. your 3500iu with 3.5mL of bac water nets you 1000iu per 1mL/1cc. at 7mL per 3500iu of HCG, you only get 500iu per 1mL/1cc.
id stick with the 3.5 per 3500. less liquid volume on the injection.