You don't convert a product.
You have 2 products: You buy product 1 in KG . This is a component in the manufacturing product 2 to produce cut product in meters. In the BOM you define that for 1 meter you need x# KG.
Waste is a challenge. In the cable business we do something similar: Every coil of raw material have a unique lot number. So when we consume, we make sure to to confirm total qty of that lot number. To make it easy, we have a development in operator screen, that show total qty og that lot number. When that lot number is used, or what is left is scrapped, the operator register all qty consumed.
You might do something similar.
Better please try to use same UOM for Purchase, Storing and Consumption.