Ok thanks - when I get near a proper computer with a decent...

Ok thanks - when I get near a proper computer with a decent screen it should make sense.

If you are researching typical cut-off grades for Zn&Pb producers then the web sites for Elura, Endeavour, MacArthur River and historic Century cut-off grades would be a good place to start, depends on how far away the mill will be from the mine and obviously as alluded to, the costs of open pit vs underground mining will also play a part. I wouldn't use S32s Cannington's costs because the extraordinary Ag grades at Cannington can give a skewed perspective. Put it this way - as an example Perilya see fit to run 5-9% Zn+Pb from their Broken Hill South Ops and Potosi and would be making money at the moment (given current metal prices, exchange rates, mines UG costs and mill very old and fully depreciated).

You are correct, most older traditional polymetallic mills always seem to have lower Zn recoveries than the comparable Cu and Pb circuits, I don't fully understand it either but my metallurgical mates always say that Zinc is a bit wierd.