If I have to pick, I prefer to know formulas, because knowledge is power! If I want to figure out something that isn't on a chart, I don't have to guess.
I do like charts, though, and make my own little 'cheat sheets' for things I need to know often. Or info like bed sizes; I don't think there's a formula for that; so I have a little chart.
So for me, best is a chart with most common usage, with the formula included for anything else.