Whenever you come across a solution to be diluted, make sure it's a water for injection available in pharmacies (well, at least here, if you can't buy syringes and needles OTC in pharmacies where you live, this won't be available OTC either).
Quarter gram of black tar is 10-15mg heroin, I can imagine.
EDIT: How come you used to inject black tar which isn't available in the UK and now you're in possession of diacetylmorphine ampules and heroin isn't used as a painkiller in the US. I know you could have migrated, I'm just asking because maybe you don't have Diamorphine Hydrochloride ampules like 5mg or 10mg. Is it a solution or powder?