You need an oscilloscope to test the output from the ECU to see what the waveform looks like and peaks at. If the signal is not up to spec then you'll get odd readings from the Stack. If the wiring from ECU connector to stack connector is fault free but you are still getting some form of signal from the ECU I'd suspect a pull up/down resistor has gone awol.
You could use an LED with a suitable resistor (something like 130 ohms if the output is 5V up to 600 ohms for 14V output) [or try a 12v led] as a quick test light to see if there is ANY output (positive to tacho feed, negative to earth) and if the output is consistent and there are no pulses missing, but if this picks the signal up while the Stack doesn't you'd need the scope to be sure.
Setting on dash is to account for number of pulses per rpm, if connected to a classic single coil/distributor setup you get 4 pulses per crank revolution, ECU tacho outputs usually only give 1 pulse per rev.