The stripping ratio is chosen based on economics.  A high grade orebody can support a high stripping ratio.  A low grade ore body cannot support a high stripping ratio.  The cost is purely driven by the cost of the mining fleet (capital and operating costs) to move the waste ore.  Larger scale operations who can maximise the economies of scale can be profitable with low grade and higher strip ratios. 

Underground operations are more expensive for capital and operating costs per tonne of ore mined but generally have higher grade ore to the concentrator due to less dilution with waste.

The other costs to consider are the cost of the concentrator, the higher the grade the smaller the concentrator which also reduces capital and operating costs.

In short there is no simple answer as the gem quality and concentration in the ore are key drivers of project economics.

The stripping ratio is also influenced by the types of overlying geology especially if you have thick alluvials overlying hard rock.  The density and RQD are important for each rock type which will also change the slope angle of your pit as you progress downwards. Note also, you may have water problems from aquifers within the upper alluvial horizons and the solid rock interface.  With the economic of mining as stated earlier, you may have to simulate the open pit design taking into the economics of moving (and maybe pumping) of water into account.  The slipping of not so solid soft sediments can also vary especially in Russia and Canada where you may have permafrost conditions as an extra worry during the summer months.  Unfortunately as stated by Justin there is no easy answer.

Generally there are three major factors:

- diamond price, mine production opex and capex (the higher price and lower costs provide higher SR) if the ultimate pit limits are assessed with a discounted cash flow analysis (DCFA).  

- slope stability affected by the strength of slope rock types and the presence of groundwater (shallow slope angles give higher SR)

-type of production schedule which affects the mine economy (very big topic) through the determination of the final pit limits with DCFA. This factor can give a  margin of up to 35% increase of the NPV which affects the SR.  

Would be happy to optimize the SR of your diamond operations without using an average SR as it is economically non-efficient.

It all depends on variables and sensitivity analysis. It depends on the operational costs and equipment efficiency, distance to waste dump etc. One of the most key factors is the grade (CPHT) and ratio of gems/near gems/boarts analysis against the market price.

Some mines are high grade ore bodies so even a waste overburden of 60m-80m does not affect the profitability of the mining operations. No one size fits all. Drilling as well can be a cost driver if the overburden are very hard overburden, this affects apart from drilling costs, machine spares due to abrasiveness which leads to high rate of wear and tear of equipment. If the mine does everything along the mine value chain, the chances of loses are very high due to lack of specialisation and technical know how.

It has been proven that in mining operations another way of cutting down costs is through outsourcing thus minimising down times and optimising production and efficiency along the mine value chain.

A lot of varied factors need to be considered in order to come with the pit optimisation limit.