This is the latest version (Oct 2014) of the Excel tool the Willem Wyngaards developed on the basis of the previous article. We call it a YAN calculator, but it is really a nitrogen addition calculator (and it also calculates yeast additions).
Yeast requires nitrogen for fermentation. If there is not enough, yeast cells are stressed and produce excess H2S—something that gives fermentations an off-odor. If you want to avoid problems, you must be prepared by adding nitrogen to fermentations.