Usually there is a way to zero the scale after you put the pan on it. If not you can measure the weight of the pan, write it down and add how much SN you want and then put that much SN in the pan. Or you can just measure the SN straight on the scale.
I suggest that however you decide to do it when you are finished wipe anything that the SN touched. It is not much but I've been told by a chemist that SN can be absorbed through the skin. You don't want to endanger others.