Just for you Bruce, page-51

Goldpirate,
The strip ratio is driven by grade in the optimisation process. The optimiser will generally hunt down the ore body with given wall angles until the cost of shifting the last strip of waste equals the value of the last bit of ore mined (less mining, processing, admin etc. costs) Therefore, in rough terms, if the last tonne of ore at say 15g/t has a value of say $420 (15/31.1034 x $1500 less costs of say $300/t to mine the ore only (no waste) and treat it etc.) and assume that it costs $10/t to mine and haul the waste, the optimiser will chase down the ore body until it requires mining 42 t of waste to mine the last tonne ($420/$10). This is not the overall strip ratio, this will be ~1/3 of this but you can see that the higher grade will allow the pit to drive deeper and thus recover more ounces but at a higher strip ratio. Some operators will not drive the pit to ultimate optimum depth but this generally depends on the reliability of the resource and reconciliation performance.
This is a rough explanation of how the design process works - in reality it is a bit more sophisticated.
DYOR etc.