For optimum running qualities, it is desirable to have a relatively consistent weight for wagons and coaches, based on their length.
For many years, the NMRA in the US has published a standard for wagon weights, which for HO is 1oz + 0.5oz per inch.
One approach for Sn3½ would be to adopt the NMRA standard "as is". There is nothing wrong with this approach although some people feel that the NMRA standard is a little on the heavy side and it is sometimes difficult to achieve the recommended weight.
I generally use Metric measurements, so I initially converted the NMRA HO standard to metric, which worked out to:
28 g + 0.55 g per mm.
By rounding the numbers down slightly, e.g. to:
25 g + 0.5 g per mm
the result is a simple formula which is about 10% under NMRA.
GH 4-wheel wagon which is 86 mm long. The "rounded" formula gives 68 g compared to 75 g based on the NMRA formula.
VF bogie wagon which is 172 mm long. The "rounded" formula gives 111 g compared to 123 g based on the NMRA formula.