Have you ever taken either before?
If not, I would try Codeine on it's own first ~100mg, and then the next day try around ~75mg of Promethazine so you can understand and feel the effects of each drug.
If you felt that one felt a little too potent, then you can reduce it, or if it didn't feel strong enough you can increase it.
Then once you've understood the effects of what you're taking, you can start to mix them - though I wouldn't actively encourage it. Mixing Promethazine with Codeine can be quite dangerous if you're not used to it.
For a newbie, I'd say ~80mg of Codeine and ~50mg of Promethazine. But we're all so different and react to drugs so differently. My honest advice would be for you to experience them separately so that you can get a feel for how they affect you. I can/have taken 250mg of Promethazine and it's had little effect on me, so I no longer bother and now use 100mg Cyclizine instead.
As for Codeine, I start to get a very bad histamine reaction at doses as low as 150mg. I don't use it any more but if I were to do so in future, I'd take 10mg of Cetirizine and 10mg of Loratadine as well as 100mg Cyclizine 30 minutes before ingesting the Codeine to help with the nausea/itching. I once scratched my head so much from the itching that my scalp was bleeding in several places - v. nasty.
