As far as readings go, running at 70mph empty you should be seeing something like 3-5psi of boost and 650-750egts. Your wastegate should prevent boost from getting any higher than about 21psi. You want to avoid prolonged egts over 1250.
For making the gauges dim with the dash lights, it is pretty easy to tap right into the wire powering the dash lights if you pull the headlight switch. When you have the switch out, just take a multimeter and see which wires change voltage as you dim the lights.