I have a microswitch on the throttle linkage and a 1/2 sec delay set in the controller so my RPM is about 2500 RPM when the system starts spraying at 40HP. Then in 1.5 sec it's up to 150HP.
The pink wire is the output voltage from the throttle body that catchuk tapped into. It's about a 0-5 volt, not a tach signal.
There are various window switches available, you can get a tach one that will connect to one of the plug coils or an analog one that will trigger off of an set voltage.