To answer your question, one would need to know your opiate tolerance. To get an idea the best way I think is to relate it to pharmaceuticals as heroin has varying strengths. If you were to take morphine pills, how much do you take to get high? if not morphine, oxycontin?
IF you dont get morphine or oxy and are in the USA, chances are you have had vicodin or percocet - how much do you take and what strength to get high.
Generalization to go by -
If you are an occasional opiate user, low tolerance, between 5-10 MG should be plenty to get you off.
IF you have a bit of tolerance - 10-20MG
Heavy users, this is different for different people, some people 50MG is plenty, others can handle 100-200mg with no problem.
Based on the question, I assume you are not an addict/heavy user. I would say inject 5MG at first to be safe. Dilute the 5mg into 20-30 units of sterile water. it should dissolve with just a little stirring, My non-pharma dope which is not 100% pure dissolves in cold water, so yours shouldnt have an issue. I will be curious to know what color the water is. Does it remain clear or darken slightly? I have very strong dope right now and surprisingly it turns the water a light brown when diluting, though it is a very light colored powder with no brown at all.