Setting sensible max-prefix limits

As I understand by now, it is highly recommended to set a max-prefix limit for peering sessions. Yet, I can hardly find any recommendations on how to arrive at a sensible limit. I guess for long standing peers one could just eyeball it, e.g., current prefix count + some safety margin. How does that work for new peers? Do you negotiate/exchange sensible values whenever you establish a new session? Do you rely on PeeringDB (if available)? Do you apply default values to everyone except the big fishes? Apart from your peers, do you also apply a limit to your transit sessions? Best regards, Lars