One further thought: Why bother measuring the flash duration? You've already got a perfectly good metric - the photograph you're trying to take. Unless this is a one-time-only situation, why not just try it?
You'll either get the photograph you want, or you'll end up with a quantitative piece of evidence that shows how far off you are, and gives you a good indication as to whether it is worth pursuing the experiment any further. Or, as some other posters to this list would put it: Don't sit around calculating theoretical numbers. Get out there and take photographs.
