Not only that, 80-90% of the "release cut" of the film is what Snyder filmed.
That math doesn't check out.
Off the top of my head there's the Batman intro(movie is 1:45ish without credits, so just that is about 5% of the movie!), Lois/Martha scenes, the stuff in the Batcave, Wonderbat scenes, everything with the Russian family, Miles Dyson at Star Labs... that is all Whedon, so there you already have more than 20% of the movie, and that's not even going into all the inserted lines here and there and the Superman CGI face stuff, so conservatively it would be at the very least in the 30-40% ballpark for the Whedon stuff.