We are using Production Batches to produce product. Occassionally rejects are made that can be reprocess into 1st Quality. Am wondering if a co-product or by-product could be used without having a consistant quantity produced each time. The amount of co-product will vary from 0 - the batch size.

Two ways in which you can handle this.
1. If you are using lot control, you could also use Assay or Potency for each lot produced to denote varying purity.

2. Use Co-product, only limitation is Co-product cannot be same as main Product. We use this method as follows:
Product A Formula for 100 units contains Co-product B(A of inferior Quality)
When A is Ok Batch completed as A = 100, B =0
When A is Not Ok Batch completed as A = 0, B =100
Subsequently use B in manufacturing of A instead of regular Materials (Purification).