Ok I'm about to mix my hcg but I'm awful at math and I can't figure out how to be able to get a injection of 500 iu. I have of liquid 10000 iu bottle of hcg and 1 20 ml bottle of bac water. I know I'm not gonna use the whole whole amount of bac water because the water will go bad before I finish my cycle. It says it has a 28 day shelf life after its open. I have more on the way just looking for help. Thanks