That should be your ported vac port. Should be no vacuum at a normal idle (700 rpm).
Your high idle likely shows some vacuum.
You'll need to use a piston stop to find true tdc #1 to set your pointer.
Info here on that.
http://www.crankshaftcoalition.com/w...op_dead_center
You can make a piston stop from an old sparkplug or buy one.