It all depends on the drop. The more drop the less cast -off required. Some fowlers seem to have too much drop for good flying shooting, but won't require cast off in consequence.
Too much is worse than straight. as noted above, 1/4" to 3/8" is about max, depending on the physical make -up of the intended owner.
There should be nothing taken as 'right' for the amount of cast -off. It all depends. :-)