Agreed, dore bars are different grade from top to bottom. A reasonable way to determine overall karat is to drill. On a small bar taking an average from six or eight small holes on top and bottom and averaging out the karat is a good practice. When using gold to finance a mining company, turning raw gold into dollars is the necessity of selling gold. Unless you can use debt to run your equipment and make payroll. Once an acceptable karat is agreed upon it doesn't make sense to pay to refine it and then sell it. Just sell it at the quality it averages out to, to the refiner.
Take your nuggets that have a specimen added value to them and market them to collectors or jewellers. Diesel, pump parts and employees don't wait for marketing to become sales. The difference between getting paid for 85 percent purity and 84 percent purity isn't worth the wait. You're splitting hairs to save a few bucks.