Maximum rate Shanon Hartley
With binary states, you can make decisions with threshold about the center of a Noise Distribution.

With 6 states, assuming equal probability of each of the states, where do the 4? 5? thresholds go?

For simplicity, assume 6_phase modulation (like 4-phase-mod but with 6, thus 360/6 = 60 degrees apart. With thresholds at the 30 degree intervals.

Now you can assume a datapoint in vector space, and compute the RMS noise power needed to move the head of the vector by +- 30 degrees left-ish or right-is and thus move past the thresholds into the phase region allocated to the 2 adjacent phase symbols.

By the way, is the SNR not +30dB?