For a differential drive system, the "center point" where all the turning calculations are based from is based on the location of all the various driving wheels. To keep the process simple, we recommend that you try to keep the wheels evenly spaced so the center is just half way between all the wheels. If the system has a caster, then you can just ignore it for simplicity.
So say you had 6 drive wheels equally spaced, then the "center" would be half way between the two center wheels. However if the wheels are not evenly spaced then you need to do some math. Say the 6 wheels were placed so that the front wheels are 7 in from the center wheels and the back wheels are only 4 in from the center. In this instance you would find the average. you could say that the front wheels are +7, the center are 0, and the back wheels are -4. If you take the average you find that the "center" is at +1 from the center wheels.
For the calculations we will say that the distance between the wheels from one side to the other is W. We will also say that the desired swing radius from the center is R, and the desired speed is V. The speed of the left side will be defined as VL and the speed of the right side as VR.