Jake
A second gauge could be a simpler screw-into-the-hole item. I have a couple of gauges on 2 ft hi-pressure hoses,and a box of adapters that let's me plumb into just about anything.Engine Tranny, Fuel,you name it.You wouldn't need anything like that.
As to reasons for such a high reading at idle, I'm gonna wait for a more experienced builder to chime in here, cuz my opinions on this matter have been poo-pooed in the past.