Look at NEC table 430.250, and find the current values for the motors. Use the 460V column. Multiply that value times 480 volts and again by the square root of 3 (about 1.732). That tells you the total KVA for the motor. Show one third of that value on each of the three phases. Do the same for the other 5 motors.
You have the right idea. I would suggest using a number that is closer to the actual square root of 3 than just 1.7. Even using 1.73 would get you a bit closer. The difference is just under 2%, so I would not make a big deal of it.