Not to mention, in certain parts of the United States (mainly the Northeast area from Washington DC to Boston), heroin is sold in small baggies called "stamps" that don't really have much scientific measurements to them. A lot of my dosing patterns were done in regards to ___ of a bag (another term for stamp).
I really have no idea what that equates to in mgs or w/e, I just know that for someone with a low tolerance, shooting up like 1/4th or 1/5th of a bag is where I'd start
I know that doesn't really help you that much but just trying to explain the situation here.
As as far as Dark Web Dope goes, I know it depends on who you go through really. Some of the stuff on there is junk and some is actually fairly good. Although, to be honest, I never had anything off the Dark Web that was better than good heroin I got from the city I live near (Philadelphia)
so, I mean, I'd venture a guess to say your stuff is decent, but not like dangerously potent. (nice backhanded compliment there)
all that being said, I feel like it would behoove you to at least snort some of the heroin first to try and gauge the strength. It is much harder to OD when snorting (or smoking, if you have that type of dope that is good for that).
Also, if you start small and try to work your way up, remember that heroin had this awful tendency to make you feel like you are less high than you actually are sometimes. I wouldn't redose any sooner than 2 hours in order to be safe until you can get a feel for how it affects you.
good luck