Hi Fouad,
I have never seen this forecast, and I am not sure I would see the value in doing so.
Normally measured weights are additionally fields e.g. in the delivery. I know customers in metals who calculated the net weight by density and dimensions, and measured the gross weight.
The data model supports this with 2 fields.
If you talk about forecasting, this is more about demand forecasting with quantities for the various paper grades, and thus derive a rough cut production plan, or a block plan. But this would be for paper grades.
Cheers, Stefan