I've painted a lot. Coverage depends on a lot of things. Is it primed? Is it already painted? Are the colors similar, or are you painting something black white? Or vice versa? What kind of roller are you using? Are you spraying? How heavy are you putting it on? What kind of stipple do you want? How good do you want it to look?
I have always found the 400 sf claims to be optimistic, and attainable only if you put one THIN coat. Even then, you get some bleedthrough of the original color due to the coat of paint's thinness, or things leaching through the new paint.
I would say 200-300 sf for figuring coverage. If it's just a stock color, like Navajo white, you can take back what you don't use.
You never know the actual coverage until you're done and do the math. And then, it's different for every job.
HTH
Steve