How to select source termination resistors by looking at signal?
Measured at the receiver end:


Undershoot means your series termination resistor value is too big
Overshoot means your series termination resistor value is too small


For low-to-high edges there is only one definition of overshoot and undershoot, so that's the one used here. I like to use a similar definition for high-to-low edges, but not all agree so be careful.