The hand held machine is the same as the scale. It is directly related to how hydrated you are, when you last ate and other factors. The idea is good, but I don't think it would be any more sound then the scale. (the scale can usually range from 22-30% depending on it's mood)

I have thought about calipers, but those can be off depending on who is doing it. Too bad I can't afford one of those water tests no clue wher eI'd even go to get one