Reference times for Tach Meters

Relatively new pilot here, trying to optimize flight time/cost for
distance flights.

I belong to a flying club that charges different rates for Tach/
Hobbs. I use the airplane mostly for longer trips (not flight
training anymore), meaning the advantage of the Tach/Hobbs difference
is mostly lost, unless I plan cruise flight very carefully.

So, to plan the flights, I'm trying to optimize flight time with cost
issues. But, to do that, I need to know what speed the Tach Meter is
operating at. Anyone have a potential resource for listing reference
times for Tach Meters? Should I look on the meter itself? In a POH?

Based on my initial research, I'm using a Tach reference of 2400 RPM.
But, if the plane Tach is a 2500, or 2300, or some other number, my
planning for cruise flight BHP, flight times, and costs gets all
thrown off.

Any help would be appreciated!

Thanks,

Jake