I believe the throttle use on start is to first ensure you're at the idle stop. Once there you don't want to be stuck there if you lose electrical power, so you roll the throttle back to just behind the stop and you can shut down even with a dead battery.
Best angle of climb is not well defined for a helicopter. For a fixed wing, it considers the vertical component of your thrust vector. A helicopter can climb at 0 airspeed, making an angle of climb of 90 degrees. For the Bell 412, if you're on instruments, keep the speed at 60KIAS or higher (its a limitation) and that will be pretty close to your best angle of climb.
Matthew.