Series termination for SPI lines

Assuming both Master and slave SPI are CMOS devices. Few things to know about are the trace length of the SPI lines between two devices, Whether the master and slave are on two different boards, and maximum operating frequency, and rise time and fall time of the driver Pins (MOSI, CLK and CSEL on the Master and MISO on the slave). Series termination has worked quite well for devices on same board with the trace length of around 6-8 inches. Assuming driver internal resistance of 15-20 Ohms (if not available in datasheet), placing a 33 ohm standard resistor immediately next to the driver pins to match with the PCB impedance has been shown very much improvement in controlling reflections. Here a PCB impedance of 50 ohms was assumed. PCB impedance varies from board to board and a fine tuning can be done by changing the value of series resistor. Care should also be taken while measuring the overshoot, undershoot and reflections as some times the ground clip of the measurement probe if lengthier, the overshoot and undershoot on the SPI lines will be magnified a lot with ringing effects too. Hence, for measurement uses a ground clip very short (say 1-2 cm) and have ground reference very close to the measuring point. Assuming 20 MHz maximum operating frequency, the high and low level duration will be 25 ns. Assuming 4 time constants, the allowed time constant is 6 ns. Hence the capacitor value can be at max of 6 ns / 33.2 ohms =  37 pF. Assuming hidden 8-10 pF of capacitor, the capacitor which is being placed cannot be more than 20 pF. Capacitor value can be tweaked for best results for the typical application.