Why not try playing around with Apple's sample code that provides a nice real-time graph of the 3 accelerometer axes. Tap on the back and see how each axis responds depending on the current orientation of the device. It should give you at least a starting point for some kind of equation.
– UIAdamFeb 23 '12 at 09:02
I try to calculate a value, which filters the taps, but on other device the value is change. Example: on iPad 2 filter.z > 1.3 then the user tap the phone, but in ipod Touch 3 the correct value is 2.5 ....
– flatronkaFeb 23 '12 at 09:29