Just for clarification...
1mL = 1 CC(cubic centimeter) = 100 units....15mL = 1,500 units, 100mL = 10,000 units, again 1 CC = 100 units = 1 mL
And I know the OP isn't talking about dope, but generally you need about 50 units per .1 of dope, I use 60-70 usually, but I know a lot of people who use 35-40 units, if I know it's fire shit I use 80 to make sure because I've always operated under the assumption that heroin had similiar solubilities to morphine and it takes 100 units of water to easily dissolve 70mg of pure morphine.(someone correct me if I am wrong)