Getting more data from your PID controller
Just knowing the steady-state error of your controller may not be enough for you to figure out how well your manual adjustment for your PID gains is working. In order to get a better picture of it you need to gather information while the loop is reacting to an input change.
However, just sending back information over the serial port while the loop is working may alter the very timing of things within the loop. So what I have done is to delay the monitoring information transfer till the loop reaches the steady state. That means actual position is recorded every millisecond and stored in an array so it is later transmitted once the motion is performed. Thanks to that I can see the system response quite nicely whether the response does have an overshoot or it is dampened properly enough. It really comes in handy when trying to set the P, I and D gains by hand.
The problem, however is that for doing this you need to have enough RAM memory and Arduino UNO is a bit short of it. What I did was to use the same Arduino code with a more powerful Maple Mini that comes with 20K of RAM so I could record the system evolution every millisecond to transfer it at a later time.