This is dangerous. We cannot comment upon a substances purity remotely. It is just not possible. You will determine the purity of your Heroin through subjective experience. The dosage of a given Opioid required to kill a person through respiratory depression is not that distant from what one desires for a good high. This is what makes Heroin such a dangerous drug. Truthfully, you might redose properly and without incident, or you might off yourself.
This is why it pays, when possible, to have a partner present when using Heroin. I've proud to say that I've saved 3 individuals from certain death using Naloxone (Narcan), simply by taking turns injecting. One of the guys I saved actually overdosed upon Heroin that he stole directly from my pocket, as it were, but that's a tale for another time. Do the right thing kids.
It would be dangerous to actually advise you on a true physical dosage. We don't know how pure your product is and we don't know thusly, what your tolerance to Opioids is like. This is just my opinion.