Hangar Flying

I just bought the Electroair electronic ignition system.  I have an O-235 with a Marvel Schebler MA-3A carbuerator.  I hear that you can pick up the manifold pressure line off of the carb, but Im not sure where and what fitting I need.  Anyone done this before or have another idea where to pick up the pressure line?  I thought about tapping a nipple into the intake manifold on cylinder 3 or 4 but I dont want to disrupt the fuel flow to a cylinder with the threaded nipple.

Thanks,
Mike