Hello all, I am quite confused about guide numbers and trying to get a properly exposed image. What I have been doing up until now is basically trial and error, b/c I don't have a light meter. I would like to do more w/ my film cameras and my strobes, but I don't want to waste film, thus I have been trying to figure out guide numbers. I am working with an Alien Bee 800 and just got a Sigma EF500 DG Super. Primarily my question has to do w/ the AB strobe. As best I can tell from the website, they report a GN of 172 using the 7" reflector at 10' for ISO 100. My digital camera only goes down to 200 and I use that exclusively right now. I read that doubling the film speed increases the GN by a factor of 1.4, thus, at full power the strobe would have a GN of 240 at 10' for ISO 200. Now I also read that using an umbrella basically halves the GN (that correct?) which would leave me with a GN of 120 at 10' (ISO 200). So if I want to shoot at f/8, I would need to move the strobe back 15' from subject. And if I wanted to leave it at f/8 and have the strobe 10', would reduce the power 1/3 from full power. Is that correct? 10.9' for f/11, and 7.5' for f/16. Are these calculations correct?