How to calculate FSK error ?

Hi guys,

Does anybody know how to calculate FSK error? I mean modulation accuracy of GFSK signal. The profile of GFSK I generated is

modulation index = 0.5;
data rate = 500kHz;
frequency deviation = 125kHz;

I have measured the signal with Agilent VSA. The FSK err is about 2.8% rms. But I want to know the calculation process behind this value.

Thanks very much~

ps: result measured by VSA