To be honest, this is something there’s not a lot of good data on. I’ve wondered about this algorithm for a long time. Were you to figure it out, you would be much respected.

The central question, essentially, is “where does the effect of charisma cap?” i.e. at what point does more charisma become meaningless? Supposedly there is a maximum number of junk items you can receive in your deliveries. When this cap is reached, you then would want to max out your luck to make sure that the items you get are the best i.e. the more venuses you get, the better.