The margins were greater than they appear to be on the video.
Given who was flying the Spitfire, I wouldn't have ducked.
At least 20 feet I believe.
Have you ever seen the pic of Ken Ellis flat on the ground at Bembridge after the same individual, P-40 mounted, came in low for the finish during the Schneider Trophy one year?
Edit: just to say that the P-40 was quite high by the usual standards of his low passes - but Ken was probably just playing it safe!