Generally unless you are a credit card you can't choose a random amount. you have to document what your material cost, how much your supplier would have charged you had you not been up to date,and the costs involved in carrying that particular job. You should be able to come up with a round about %, say if your vendor charges 3 % on all purchases carried over month to month. Add your own 3% to that number. Basically have a number that you can document and justify
