I think I'll be snorting I looked into IV it kind of seems easy but still difficult because if I do miss a vein it'll go into my skin which will still get absorbed but not as effective.
IV is immediate while snorting will take a few minutes to start the effects. You can squeeze really hard to see your veins pop up if you struggle seeing them.
If you trust the guy and he told you it's really pure fentanyl then I think 15mg might be enough but it's always safer to get more just to compensate. Street fent can always range in purity.
Read this from the DEA :
It says domestic is usually 14% pure. I'm assuming they mean here in the U.S. That means if you got 15mgs of 14% pure fentanyl, do the math :
15 mg x 0.14=
2.1 mgs of that 15mg is
fentanyl while the rest
12.9 mgs isn't.
So the other 86% is just a bunch of fillers or other cheap drugs to add in weight.