OK, now I know what you are after. The way we do this is the ECU provides a 5 volt level to the TPS (Throttle Position Sensor) which outputs a linear voltage proportional to throttle position, so what we do is use a small simple comparator circuit. I'll try to get a circuit done later today. You want nitrous when the TPS reaches about 3.7 volts correct? Anyway, you want a simple small comparator circuit. The comparator will drive the transistor which will turn on the relay when nitrous is called for.
Ron