Trial and error is the way to find out as your body fat content affects your density and someone the same size as you may need different weights. In the shallows try different weights by adding them on until you float vertically with your eyeline on the waterline and breathing in and out shifts you up and down marginally, then you should be right.
Here is my weight belt, but I wear a 7 mm wetsuit because the water here is cold, so I need plenty of lead. The home-made weights are about 3 lbs each.