Manufacturing issue (produce material wight can't be fix , in that case how to create BOM )

Hi, i have plastic sheet industry. I create plastic sheet on client requirement. Plastic sheet production require chemical, granules , heating and manual cutting operation. Our production standard tell if we supply specific amount of raw material, chemical, heat and cutting operation then we will get 300KG roll. And we assign roll no for each roll. Roll no is unique id for tracing roll. Roll No consist all details like roll weight, length, raw material details etc. We sales roll to client with roll no identifier.
But when we will get final product, it weight may be more than 300KG or less than 300KG and some time 300KG. Because it all depend on chemical and heating process.
So we can’t fix BOM because output can be differ. I think weight entry of produce item should be changeable. Please let me know how i can handle this issue?

In ERPNext version 11, you will be able to make manufacture entry which will not not tightely validated with the BOM. For how, you can try if Backflush based on actual qty helps. Setting available in manufacturing settings.

You can use Batch inventory to track Roll. Or you can also create another item for role. After production in KG, create a repack entry for conversion from KG to Roll.