Mike,
Sorry for the delay in responding. Had to dig out the correct reference.
You are correct that the basic letter rate (<20 g) to the U.S. was 25 Pf. As to the airmail rate...
The Michel postage rate handbook for Germany includes airmail rates to 1933, but after that, it defers to reprints of the Luftpostlisten -- the books the German POs used to determine airmail rates.
Philabooks secured the rights to reprint the Luftpostlisten from 1933-1945, and I have a copy of the volume that covers Aug/Sep 1939 to the end of WWII. If Philabooks ever comes back online, I need to grab 1933-1939. If you see one for sale, you can't miss them -- the font is horrific!

Looking at 1 Feb 1940, this is what it says.

If I'm reading it correctly, the base airmail rate to New York was 40 Pf., with an additional 10 Pf. for any further routing within the U.S.
And that works with your letter -- 25 Pf. base foreign rate + 40 Pf. base airmail rate = 65 Pf.