A 0.69 surface acre round pond that averages 9 foot deep will have 2,031,310 gallons of water in it. Using Vertex's Lift Rate Chart...
..you can see that a single diffuser (XL1) at 8 foot deep will move 1,250 GPM, or (x 60 minutes in an hour to get) 75,000 GPH, or )x 24 hours in a day to get) 1,800,000 Gallons per Day. That is shy of the rule of thumb by 10%. The above chart is based on 1 CFM through the diffuser.
Some folks will try to design their aeration system to turn the pond volume more than once a day, maybe 2 or 3 times. This would allow the system to be put on a timer thus reducing the run time per day, but still getting at least a full turn over per day. It adds some flexibility at the cost of a larger pump or multiple pumps.
With your deepest diffuser at the 8 feet we have been discussing, the pump pressure would equate to 4 psi (thereabouts) and from the above chart the pump would be pushing 3 CFM. That's enough for 2 or 3 diffusers. This would double or triple (respectively) the amount of water movement. That's key.
Keep in mind that once you add more than 1 diffuser, some control valves would be advised so that you can adjust air flows which effects the bubble boil sizes. Without the valves and with diffusers at different depths...the bubble boils will be larger at the more shallow diffusers. The valves at the more shallow diffusers could be closed off some to force the air to the deeper one. Once, again...more flexibility!
EDIT: There is a fair amount of design work that can go into an aeration system. It is very important to use the appropriate sized air lines and to not restrict air flow with improperly sized valves and fittings. A topic for discussion later on in the thread. Lunch break is over.