well, since the DFHT is most decidedly not a BB workout...... and a 5x5 rep scheme is not either..... Remember that you can never truly isolate the chest anyway, you will always use triceps and some front delts. Biomechanically, I don't see how you can use lats....ever, but I could be wrong. It is true that you want to have a good base on your lats to press from if that is what you are thinking of? As far as angle of the arm, I'm not certain that anybody can press any appreciable weight with arms perfectly perpendicular to the body. I personally use th "PL" form, and reccomend you do the same.
Pax,
GS