Modulation Index for frequency modulated transmitters is given by
Where 
is the Modulation Index
D is the peak deviation (1/2 difference between maximum and minimum frequency)
m is the modulation frequency in hertz.
This material is presented in the ARRL Handbook in the Modulation Sources chapter.
The interesting fact is that when the modulation index is 2.4, 5.52 or 8.65 the amplitude of the carrier vanishes.
As an example, if you modulate an FM transmitter with a 1200 Hz sine wave and slowly increase the amplitude of this modulating signal, the carrier will disappear when the MI is 2.4. Then the deviation is equal to 1200 x 2.4 = 2880 Hz. If the modulating frequency is 2200 Hz when the carrier vanishes, the deviation will be 2200 x 2.4 = 5280 Hz. When you increase the amplitude of the modulating signal the carrier will reappear until it vanishes again at 5.52 MI, etc.
But how can you determine when the carrier amplitude goes to zero? A simple way to do this is to use a CW receiver tuned to the frequency of your FM transmitter. The procedure is to key the FM transmitter with no modulation. Tune the CW receiver to hear the carrier. Use the smallest possible transmitter power into a dummy load. Set your receiver to minimum rf sensitivity. When you hear the carrier from the FM transmitter, apply a small amplitude modulating signal. Tune your CW receiver to maximize the tone of the carrier and minimize the modulating signal. Slowly increase the amplitude of the modulating signal. If your modulation signal level control is not too coarse, there should come a time when the carrier vanishes. When you reach this point, you know what the deviation is - according to the equation above.
The mark tone for packet is 1200 and 2200 Hz. for space. When you use the CAL command (included in the terminal program used for TNCs) you can select either frequency. Some TNCs such as the newer Kantronics models have digitally adjusted level sets. This makes the adjustment even easier. Instead of using a screw driver to adjust a potentiometer, you click on an up button and you can see a number increase as the amplitude increases. Stop when the carrier disappears. When the carrier first vanishes, your deviation is 2.4 times the frequency of your modulating signal.
In the example give above using the 1200 Hz tone, the deviation would be 2880 Hz. Suppose that you wanted to set your deviation to 3000 Hz instead? You can do this if we assume that the deviation is linearly related to the modulating signal amplitude and that the deviation is zero when the modulating signal is zero volts. These assumptions ought to be accurate enough to permit some degree of interpolation or extrapolation. You can set up deviation values fairly near the one determined when the carrier vanishes by using a suitable equation.
Suppose that the carrier disappeared when you had 166 set on the amplitude digital level indicator. You could also use whatever the actual voltage of the modulating signal was at that time.
So a setting of 173 should represent a deviation of 3000 Hz. Other deviation settings may be determined by using the same procedure.
There are two cautions to be aware of. The digital level adjustment in the Kantronics unit is not linear. For values below 256 the voltage change is 0.5 mV per step. At 256 and above the voltage change is 15 mV per step. This step size may be too coarse to hear the carrier null.
The other fact is that 9600 baud packet operation does not use tones, so this “poor man’s deviation procedure” can not be used.