~50 % is the right settings. The car should be on operating temp., do not need to unplug any vaccum line. It sounds like you are ok, but if any adjustments need to be done, an interval of 10 - 20 sec is advisable. Use a 3 mm allen to do the adjustments.
Is there a problem with the car or just playing with your new $ 11.00 sears multi-meter
I hope it helps.
Good luck.