as has been said, we can't tell you how much dope will get you off, if for no other reason than we don't know the purity of your dope (and you almost certainly don't either - unless you're a chemist, but in that case you probably wouldn't even be asking).
I don't mean to be a dick, but there is only one bit of advice anyone here can really give: start low with the dope and work your way up. ESPECIALLY if you are going to IV your dope. I would not recommend starting with more than 1/4th of a $20 worth of dope. but even then, if it was really bomb dope and your got a good deal on it you could OD, even on that small bit, even with your opana tolerance.
frankly, if you're trying heroin for the first time or so, and therefore don't have a good feeling for how good your dope is/how it will affect you, I'd HIGHLY recommend using the heroin intranasally or smoking it. that way you have MUCH less chance of OD and can EASILY titrate your dose. Once you've figured out how much will get you off using it intranasally or smoking it, don't IV more than 1/4 that amount for your first IV shot. might be a little weak, but you can always take another shot.
trial and error here OP. no other way. and because of that it's especially dangerous, and likewise especially important you start SLOW and work your way up. if you make any mistakes and try too much IV for your first time, you're asking to OD. and accordingly there is a very good chance it could lead to serious complications/death, especially given your new to heroin and probably don't have narcan or anything handy or use dope with people who do (and know how to use it).
it's better not to get as high as you wanted today, considering it will give you an idea of how much you need to safely get high tomorrow (or even later today!). understand?
BTW, don't use SWIM. it doesn't help you legally speaking and is against the rule/standards of BL.
