Distance from floor channel vertical secondary air tube and port entrance

Working on my BB RMH and wondered what the recommended distance is from the floor channel vertical secondary air tube and the heat riser port entrance?

Many thanks for any help.

Hi John;
My secondary tubes have the 6" stub set back apx 1"
The tube itself is even with the back wall.
So I would say 3/4" -  1.5"  would be acceptable.

Thanks very much Thomas - I'll give that a go!