What you heard was true!
As for the Combat Kan with an OS.25fx
MA 9x4 prop start by sliding the can until it touches the end of the pipe.
Back it off 1/8" start there and tach it. With 15% nitro use should be in the 18k range on the tach
If you can hold the plane in a cradle at wide open throttle you can tune the can with a rag by turning it in or out and check it with a tach.
Webra 25 some pilots trim the props a little to get them on the pipe with higher rpm. Again you may have to put in the cradle and tune the pipe.
Hope this helps