Like cooking food, following a recipe strictly is a very good idea at first. But, with experience, tweaking, tasting and fiddling around seems inevitable. Just from years of experience with developing and printing, I can do many things intuitively now that would have been risky or disastrous early on. Still, I always keep notes on a legal pad. Toning, however, is always done by eye. Haven't figured out yet how to be empirical about it since there are so many variables, and so many nuances that depend on the image.