Are there any published standards to refer to when negotiating net working capital with a seller?

I am in negotiations to buy a business and the primary sticking point is net working capital. The seller believes there should be no adjustment, that any capital tied up should be theirs. As a financial buyer, I have modeled and set price based on a multiple assuming normalized working capital will stay with the business. In this particular transaction, the expected net working capital represents ~12% of the sales price.