If you are not a regular user of other opioids (preferably pharmaceuticals like oxycodone, hydromorphone, morphine, even codeine or tramadol) don't bother with heroin.
In fact most "heroin" these days has a good chance of containing no heroin at all. Fentanyl or its analog carfentanil are more cost-effective and subjectively "stronger" (plus shorter lasting) so they have overtaken heroin as the active agents in street "down".
Because there is no standardization of street heroin, it is very hard to tell how potent your material is. It could be anywhere between 50% fentanyl to less than 1%, and could contain carfentanil as well (in variable amounts). So a lethal dose could be from less than 4mg, to 200mg or more. Oh, and to add insult to injury, some unscrupulous dealers even add benzodiazepines to their "heroin", leaving their customers blacked out for hours (and making OD's even worse to treat)
I would see if you can find someone else who uses the same "down" that you are buying, and have them show you how much they use. Fentanyl is almost always smoked off foil where I'm from (as it's more effective and more rapid than snorting). If you can't find anyone to show you, then start with tiny amounts, much less than 1/4 of your dose, and make sure to wait in between doses. Preferably have someone with a naloxone kit nearby too.