all depends on what currency your mining 
for example in BTC;
in AF make sure you have the "sync similar gpu option" UNCHECKED
keep the voltage at stock, push powertune all the way up, drop the vram and far as you can
push the core clock in increments of 5 and test for 12-24 hours. if the gpu is stable continue to push addition 5 and repeat until your GPU crashes then you know your cards clock. you can then start to drop the voltage little by little testing yet again 12-24 hours until your card crashs
OC testing is rigorous and time consuming. usually requires a lot of babysitting your rigs. but once you find the sweet spot your rig is pretty much automated
for example in BTC;
in AF make sure you have the "sync similar gpu option" UNCHECKED
keep the voltage at stock, push powertune all the way up, drop the vram and far as you can
push the core clock in increments of 5 and test for 12-24 hours. if the gpu is stable continue to push addition 5 and repeat until your GPU crashes then you know your cards clock. you can then start to drop the voltage little by little testing yet again 12-24 hours until your card crashs
OC testing is rigorous and time consuming. usually requires a lot of babysitting your rigs. but once you find the sweet spot your rig is pretty much automated