Connect it to the main bus. That way when you turn on the master, you can read the battery voltage. You also want the screen connected to the main bus, not the avionics bus. Do not connect it to the avionics bus.
As soon as you turn on the master switch, the screen will boot up, and you will have everything you need to start the engine, voltage for cranking, fuel pressure for starting, and oil pressure after it starts.
Then turn on the avionics bus for the rest of your avionics.
Good luck.