I have absolutely no justification for 10 or 25 other than 25 seems excessive and 10 seems less excessive.  

Eventually (not in this PR), many of our receiver drivers should be split into a common (protocol) portion (in PiOS/Common) and a F1/2/4 specific piece that deals with the timers.  That would consolidate the protocol related defines into a single .c file under Common.  The scope of this define really is within the protocol definition for PPM so I'd like to keep it in the .c file(s) until the refactoring is done properly.