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It is pretty much trial and error. There is a fairly wide range of stability among CPUs that chip manufacturers rate the same clock speed. So let's say you have two phones that are the same model, same ROM, and the same 1Ghz CPU. One of them might be perfectly stable at 1.4Ghz, while the other may crash more often than a NASCAR race at 1.2Ghz. For the most part, it is luck of the draw. The way I do it is just start ramping up and running various apps as an informal stress test for an hour or two to see if it is stable, and if so step up a little more. I repeat this until I start getting stability issues, then back down a notch. The app that seems to cause stability problems first is Gallery, so I suggest you open Gallery each time you change the clock speed and use it as your Android canary in the mine. When Gallery will work still work fine without force closing or causing reboot, you are probably good.