You have no point to flee from.... Brennan didn't need to know the distance from the floor to the window sill..... He said that he saw the man STANDING UP while aiming the rifle out of the window and he could see the entire upper portion of the man's body from his belt up.
Brennan's observations re anybody standing are handicapped given his assumption that Jarman and Williams were standing

The sniper window was open to a height 1/2 of the distance of what the above windows are here.
Oswald would have to be kneeling to be able to have his shoulder against the window sill