Thanks Nick,
Your confirmation of the theory requires that I do additional research!
I am working on a lift fan for a slow airplane, the drag is not a big factor in this case.
So far, I think the design requires a large bellmouth inlet, and the largest possible diameter two bladed prop and a divergent outlet duct around 20% of fan diameter as you suggested.
I think a wide blade tip might help reduce the need for a close clearance, but I have no data about tip clearance yet for a shrouded prop. I think the ducted fan information does not apply.
This is interesting! Double the thrust!
slowrotor