How do I determine the minimum responsible weighing mass on my balance sheets?

Minimum weighing

d = Reading Unit Minimum Quantity to weigh   Relative error         0.1 mg                   10.0 mg   0.2 mg at 10.0 mg = 2%           1 mg                   100.0 mg 2 mg at 100.0 mg = 2%       10 mg                     1, 00 g   20 mg at 1.00 g = 2%

Ideal is provision at the workplace: on the basis of the sum of the mistakes made, the mass is to be weighing at least = 100 s _w    

S _w   = standard deviation of 20 weightings of windcatching object on the balance sheet under practical conditions. When determining each weighting completely from scratch, ttz rebalance to zero.......