Injecting fentanyl citrate with tar heroin dosage?

[StreetSpirit86]

SWIM usually injects about . 3 grams of tar heroin in one shot (IV) They came across a vial of 100mcg/2ml fentanyl citrate solution. They have used heroin laced with fentanyl before many times without an accidental OD, but never actually added the fentanyl to their heroin themselves. They want to know what a good starting does would be. Since. 3 grams is 300mg of heroin (for medical grade anyway who knows about street heroin, but. 3 is the shot they do) , and 100mcg is only .1mg, what is the suggested dose to add in order to feel it but not die?