"FPV is very, very usefull when encountering unusual attitudes. You can put the FPV on the horizon and the airplane will fly level.
The optimum difference between the FPV and airplane attitude is 5 degrees. If it is more, you need to select flaps."
I just commented on the 737-800 high altitude stall recovery thread. This post is an example of techniques that work in some events but won't work in all events. If you experience a very high AOA event putting the FPV on the horizon WON'T NECESSARILY reduce the AOA below the stall AOA or perhaps won't allow the aircraft to fly level.
"The optimum difference"? You need to select flaps prior to the minimum flap speed. There is no "optimum difference".
I have about 2500 hrs with Boeing AOA gauges. Using the FPV differential is less accurate than using the AOA but we still used the minimum flap speeds which is Boeing's procedure.