CJA Marketing Channels - Persistence setting on derived field?

Hi,

Would like to seek advice on marketing channels using derived field in CJA.

The marketing channel derived field is done with the function template, however, most events are grouped under "Direct" in workspace report which was not expected. I've changed the Persistence as "First Known" as it looks good. My question is whether this is the appropriate way? or is there other considerations or practices on setting up marketing channels with derived field?

Thanks,

John