Well what I can give you is a subjective experience that it enhances slightly the effects of codeine. I believe the method of enhancement (CYP450 stimulation) would be the same for both codeine and hydrocodone. For codeine I found that it made it just a little stronger (maybe 120%) but shortened the duraction. How much of that is placebo? I have no idea.
Anyway you don't have enough to OD unless you pop a bag of the promethazine. It is just a special antihistamine and isn't particularly dangerous and won't interact with the tylenol. Just take one or two pills of prometh with a couple vics...your only danger really is the anti-histamine putting you to sleep.
i used to take between 150mg and 250mg(6-10 tablets) of prometh,along with whatever opiates i was using at that time.
From my experience,i found a tolerence to them very quickly,but they did there job.