What is a maximum g force value possible to simulate with a phone in a hand?

I am writing a simple f1 g force meter for android using accelerometer. Everything looks fine but the only problem is that the maximum value I can simulate is 2.85. Probably I do something wrong because it is simple to "make" for example 2.5 g, but using all my strength I can't have 3g.

That is how I calculate g value:

double x = event.values[0];
double y = event.values[1];
double a = Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));
double currentG = (a/ SensorManager.STANDARD_GRAVITY);

Any help would be greatly appreciated! If you need I can add more code.

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