Funny cars off the starting line

I took this picture of funny cars off the starting line at the Orange County International Raceway in Southern California back in the 1980's. I had my first medium format film camera, the Mamiya RB 67, and I was sitting at the top of the grandstand. The distance from my position to the track was too far for my flash to illuminate the action. There were photographers down on the track, though, so I used their flashes for my picture. The question was, however, what f/stop to use. I made an assumption that the guide numbers of their flash units the other photographers were using was 120. If that was true, the formula to calculate the f/stop was GN = Distance x f/stop. So, if the GN was 120 and the photographers were about 15 feet from the cars (another assumption), the aperture would be 8 (15 x 8 = 120). I chose a one second exposure so several flash units would fire during that interval. It turned out to be a correct calculation. I even captured people in the audience on the opposite side of the raceway shooting with flash (which would have no effect on the cars). You can see that the stagecoach car in the foreground was illuminated by flash twice by two different photographers during my one second exposure.