How do you calculate working capital for a mining company?

Hi all,

I have recently came across the mining company and wonder if common working capital forecasting methods (% of COGS/Sales or days outstanding) would still be appropriate? Or is there is a better method?

Explanation for my question: Inventory reported on the BS of a trading firm, distributor or even a processing firm would be not too far from the market value. Changes in inventory level hence works well in estimating used/released cash.

However mining company's inventory typically consists of unprocessed ore reported at cost which is often not reflective of the market value of metal contained in the ore (even adjusting for milling costs, royalties, etc).

Any opinions / suggestions?