Fish feeding is one of the most complex issues each of us deals with...how much? how often?
Different fish have different needs...some need more frequent feedings then others.
Not only is it important to me that my fish are comfortable and thriving, but I need to feel the same.
My wife is always saying "Your fish look hungry!" --- I say, "that is their job all day long"
As for how much? --- guesstimating - but enough so it looks like everyone ate something, and little left over after feeding frenzy dies down.
I like to mix up their food also, not so much because they need it, but because it makes me feel better that they are not eating the same old thing every meal...also there may be some fish in there that have special diets I know nothing about...so taking no chances.
I personally would like to error on overfeeding instead of underfeeding. I have crabs, shrimp, protein skimmer to take care of excess...and if I ever get an algae bloom, I would slow down.
But I certainly don't want to be running a concentration camp, where fish are only getting enough to minimally survive on.
Guess you just learn over time what your tank needs and whatever makes you feel comfortable also.