I'm trying to get my head wrapped around the idea of using guide numbers to get a good starting point when using small flashes. The thing is that I'm trying to find a way to do it quickly, in a few seconds tops, and my head is starting to explode. Here is the scenario. I usually have my ambient exposure down, let's say f/2.8, 1/30s, ISO400 and this is fixed. I also have the distance set, let's say 2m. The flash has a guidenumber of approx 30m at 35mm zoom and ISO100 but it's set at 14mm so I have to account for that. Now what power do I need to set the flash to (everything from 1/1 down to 1/64 in 1/3rd stop increments is possible)?