Speak to VDO and they'll advise of a calibrated sensor you can use with the gauge.

Ref having a manual on/off switch for the fuel pump, just a heads up but you're basically circumventing the safety feature the original relay offers. In the event of a crash when RPM is zero'd, the relay terminates power to the pump preventing fuel pumping out everywhere if you split lines etc. But taking that out of the equation and having a manual switch you run the risk of the pump continuing to run in the event of a crash.
People really struggle to wrap their heads round these relays for some reason but they are very simple and the aftermarket equivalents are just as good in my experience