You need very little accuracy in those measurement because you have a very accurate proxies that you can measure. The amount of food you eat and the weight on t

You need very little accuracy in those measurement because you have a very accurate proxies that you can measure. The amount of food you eat and the weight on the scale. When the weight doesn't go down, eat less and move more. The only thing you have to do to be able to do this is tracking the amount of food you eat, for example by keeping a food diary and using a kitchen scale.