Ann: Robust Maiden Resource and Positive Scoping Study Kookynie, page-39

From my laymans perspective Due to the main factor that strip ratio increases exponentially as depth increases linearly , At some point at depth AIC of open cut goes higher than AIC of UG that theoretically would be the point to start mining from underground
there are other factors to obviously apart from strip ratio , , ore body dip , vein width rock competency that are either positive or negative factors for OC against UG .

they seem to be using a depth of around 120 meters for the pit , I had used 90 meters , personally feel they will go back to a lower depth once they do a DFS , for two possibly three reasons .
1.They will find that they have used too cheap a price for moving dirt (BCMs) but their underground costs look about right (except that they will need to push their drives/levels closer together )
2 The deeper a pit especially in colluvium the greater the chance of pit failure as the pit circumference obviously increases at surface , and they will end up having to use a shallower repose angle to compensate which also pushes up costs and even then the risk of pit failure still increases more with depth and I doubt they have taken that risk into account in a simple Scoping study
3 Maybe To mitigate the repercussions of pit failure they will move to a hybrid of a boxcut and spiral ramp , go down 30 meters on a straight box cut and then spiral to the portal as easier to repair a boxcut than a 90 meter pit wall failure that is used as the ramp , also tire wear and rim pull is less plus is a low cost truck duel passing zone ? Something they could at least look at .

Page 3 of the Cube report shows going from a 23:1 Strip to a 32:1 strip pushes up costs by $400 an oz , and we know strip ratio increases exponentially to the factor of depth ,
From that you could work out what the cost for every increase of 1 amount of waste for the same amount of ore taken out maybe (400/(32-23) =$44 per 1 increase in strip , that does not take into account changes in SG or rock competency for pit wall angle

I guess you can play around with the formula for a cone to see that think it is( Pie * R squared * height) * 1/3 and look how volume changes exponentially with the cone height .