Formula for adding Multiple Flash Pops

I know this is a bit out there but I was discussing it with a friend earlier today and am still looking for the answer.

So in the video posted below, how would you calculate the flash exposure?

http://www.youtube.com​/watch?v=CtMQtAz250c

Now I have an extremely good understanding of how flash exposure normally works, calculating ratios, flash vs. ambient, etc. But when adding up multiple pops in a single frame, how do you calculate it?

Is it essentially, if I meter a light at f/2.8, two pops of that will equal f/4? I do have an L-508, but I'm pretty sure it doesn't cover something like this.

And in the video is it just a case, of hoping there isn't too much overlap, and not worrying about blowing the highlights, as they should pure white anyways?

If anyone has some insight or experience with it please let me know,
Thanks

I would use the Cordless flash (cumulative) mode on my Sekonic L-358.

Or the auto mode on my Sunpak 555 set to the proper f-stop and ISO.

Johnsoir wrote in post #9471799
I know this is a bit out there but I was discussing it with a friend earlier today and am still looking for the answer.

So in the video posted below, how would you calculate the flash exposure?

http://www.youtube.com​/watch?v=CtMQtAz250c

Now I have an extremely good understanding of how flash exposure normally works, calculating ratios, flash vs. ambient, etc. But when adding up multiple pops in a single frame, how do you calculate it?

Is it essentially, if I meter a light at f/2.8, two pops of that will equal f/4? I do have an L-508, but I'm pretty sure it doesn't cover something like this.

And in the video is it just a case, of hoping there isn't too much overlap, and not worrying about blowing the highlights, as they should pure white anyways?

If anyone has some insight or experience with it please let me know,
Thanks

The answer depends on whether the multiple pops overlap or not. If you are flashing different parts of the scene with no overlap, then the total exposure would be the same as a single pop exposure. However, if the pops are to the same area of the scene, then the pops are cumulative. So your f/2.8 pop becomes an f/4 with two pops, f/5.6 with four pops, etc.

If you don't want to figure out the total exposure of the multiple pops in your head (although it's not all that hard to do), you can use the Multi mode feature of your flash meter to figure out the exposure for you.