Discerning data ranges

How do I know when to use what for any given command?

E.g., -i expects [0,255], so I would have to do -/ 257 to a 16-bit image, whereas -blend_fade expects one of its arguments to be [0,1] (Using -blend_fade - #3 by David_Tschumperle).

ATM, I am learning by trial and error and asking questions.