Thank you for your response.
I have Assasin / Ritualist / Dervish characters who are quite capable of solo farming.
But when the PVX Wiki build page for "Dervish Chalice Farm" reports "20 runs - no chalice drops" then something is wrong with the actual drop rate.
I understand that bot-farming is bad for the game, so I will go and see what real player farming builds get from the CoF run.