That seems extremely long to me - for reference, my 16" Butet with a 1 flap was 13" long. My friend's is a 17" with a 2 flap and I believe hers is 14". My current Childeric is a 17" with a 2 3/4 flap (forward) is under 13". When I was shopping, at least, the average 16/16.5" saddle had a 13" flap unless you had one that specifically long or short.
Lots of used saddle websites will list dimensions for you. CWD, highendusedsaddles.com, and Maryland Tack Exchange all list lengths right on their websites.