It's been a couple of years since I last participated in a flour-bombing contest, but my bombardier and I did pretty well. We came over the field at about 70 mph and 50' AGL, 10 flaps, and he opened his door (no opening window on that side) to drop the flour bombs. We were late in the game, so we had a pretty fair idea when to drop, and he did it really well.
Here was the interesting part--the kid was 16, had never been in a small airplane before in his life, and when I told him that he'd have to open the door, he didn't seem worried at all, unlike many passengers who think they'll fall out. Of course, he found out that it took a pretty healthy push, even at only 70 mph, to get it open enough to drop the bombs. Oh, and the reason I didn't switch seats with him so that he could use the window is that I hadn't flown from the right seat in many years, and I just didn't feel comfortable doing it without some practice. There was a time years ago when I was more comfortable in the right seat than in the left one, but that's like 30 years ago.
There is an old "rule" about flour bombing: If you're called upon to judge, just stand in the middle of the target. That's the safest place to be because it's least likely to be hit. That has been true at every flour bombing contest I've ever attended. At one I attended, there were several kids standing in the target area, and as the contest progressed, the bombers were getting more accurate. One kid decided that they were getting too close (the closest was still probably 20-30' from the target circle), so he ran outside the target--and you guessed it, the next bomb got him directly!
They're a lot of fun, but like all such things, it's really important to remember to fly the airplane. Obviously the slower and lower, the more accuracy, but pretty soon you're getting into potential trouble. I chose 70 mph, because with 10 flaps, that's somewhat above my normal approach speed, so I had plenty of control, yet it's slow enough for a reasonably good bombardier to get a feel for when to drop. I chose 50', because that was enough to comfortably cross over the power wires at the end of the strip without having to climb at all, although I did climb as soon as each drop had been made.
Cary