What's your opi tolerance like?
I'd start with 20mg or 25mg promethazine (depending on whether you have Phenergan or Sominex) and wouldn't advise any more than that unless you're experienced with this kind of combo. Even if you're experienced, it can still be hazardous.
Having said that, I was taking reasonable doses of #4 heroin with 40mg promethazine (plus etizolam) the other weekend, and I'm alive.
Just don't throw booze into the mix.