Michael Yen has developed a simple chart to calculate the distance of the flash using the guide number of the flash, the idea is not new and online calculator is also do it automatically after entering the required values, now the question for many beginners is how to find the guide number of a flash? so take a example from canon Speedlite flash, for example the 430EX has a GN 43, 580EX have 58, 600EX have 60 GN and 270EX have 24GN.