Calculating Required Number of Fetch sessions

[Jamie <ja...@aol.co.uk.INVALID>]

Hi All,
Is there any formula for calculating the number of required fetch sessions?
I notice we have a lot of "fetch session doesn't exist" messages in the consumer logs, my understanding is that this is caused by fetch sessions being removed by the broker because we have more consumers than fetch session cache slots.
Is there a formula for working out the number of fetch sessions required?
Maybe: num_replica_fetchers_per_broker + (num_consumers / num_brokers) 
Many thanks,
Jamie