To try and equalise this I try and get wagons to weigh roughly the same.
To this end, one birthday I asked for a set of scales. These, by Salter, are fairly sensitive and are ideal for the job in hand.. Usually I place the finished wagon on the weighing pad and add lead to get the weight to about 18 grammes.
Some wagons need a lot, Brass kits especially. But loaded they may only need a little.
I find that similarly weighted wagons run better especially when shunting.