Shit happy I learn't this now
I can't vouch for dex and heroin, but seeing as this thread hasn't gotten you many answers I will post for what I do.
I do Hydromorphone + Methylphenidate in a 6:5 ratio.
And Morphine + Methylphenidate in a 6:1 ratio
This I do IV. Please note that opiates are my DOC, I like a more sedating high but I find the methylphenidate (MPH) adds to the rush/euphoria. For this reason I use a small relative ammount of MPH. Most users if wanting a true speedball (or alternative) would probably raise the ratio of the upper a bit.
Roughly adjusting for heroin and dex assuming heroin is 2x morphine potency and dex is 2x methylphenidate potency then you end up with the same ratio 6:1. Then you would have to adjust for bioavailability which I will do in a bit.
Edit- Neighborhood beat me to post so this may be invalid but I will continue.
Adjusting for intranasal absorption. Heroin BA = ~53%; Dex BA = ~75% -
basically end up with a ratio of 7:2. That's what I would try (obviously in a small amount first) if I was doing what you are doing. Of course if the dex is pharmaceutical grade and the heroin is off the street and not pure you may need to
adjust for the strength of the heroin and add more (hence why I stick to pharmaceutical speedballs).