This is a general comment as I haven't a Dash to set up either...
Whether the signal you are talking about is an output from the ECU, pulses on a wire you have installed to an ignition component or injector lead, data in the serial stream from the logger, or even the output of a sensor you have fitted directly to the crankshaft, the signal is not "RPM".
It is frequency that depends on RPM (is proportional to), which you have to scale to make it a sensible revs reading.
As you have identified, the scaling factor depends on "pulses per rpm".
The best you could hope for, because the pulse is coming from a 'clever' ECU, rather than a 'dumb' wire, is that there is
one pulse per RPM. This is possible but not probable.
It is more likely that the tacho converts the
Hz into
RPM.
Ask yourself, "What is there inside the logger that does this conversion?"
It is the display device, whether it is dash hardware or our Analysis software, that we instruct (with the scaling factor) to make the conversion.
This is necessary for both display of sensible revs and to correctly trigger a shift light.