I prefer to measure the depth with a tape or yardstick. It's easy to calculate a conversion factor for most any kettle. I know the volume beneath my false bottom and that constant is easy to figure in. You can fill the kettle with a known volume, measure the depth and back into the factor as a cross check. I don't worry about the ribs in the keggle and just assume it's uniform. Minor difference IMO. I always have a pocket calculator handy, so using the factor is very easy. I'm sure marking the inside of a pot would also work too, but I don't know how easy it would be to read the marks. A lot of guys use a marked off dip stick. Those are easy to make too.