Costing BOM and Mixed Costing

A product which is manufactured in-house has more than one production version.  Each production version has its own BOM and Recipe.  To calculate a blended standard cost, we will use mixed costing functionality and assign ratio to each production version.  However we want to copy the BOM usage 1 (production) to BOM usage 6 (costing) and make changes only for costing purpose.

Hence each production version will have a BOM for production and equivalent BOM for Costing as follows:

If a process order is created with Usage 1 /Alternative 1, how does the system know to compare actual cost with procurement alternative with Usage 6 / Alternative 1 for the purpose of variance calculation?

Thank you,

Siva