Hi there,
I have a scale like that and I remember Roe and a couple of others saying they are not that accurate. I think they are good for getting a general idea of which direction you are going, but they measure electical resistance and are affected by how hydrated you are.
What I do to keep it as accurate as possible is always take the reading in the morning when I first get up. I think the reading is higher than it should be because I'm not well hydrated but since it is always at the same time I can see which direction I am moving.
I know if I take it before supper or in the evening the reading is quite a bit lower - because I assume I am better hydrated at that time of the day.
HTH,
Punky