carburator dry flow bench Holley 500cfm testing

My home made dry flow bench is simply a 5hsp shop vac powered thru a 1 inch id pipe that turns up 90 degrees thru a (too small) plenum right under the table top with a 1.69 inch hole (adjustable) to fit most carbs individual base throttle holes. The 1 inch runner pipe is totally controllable: off & all the way to suck all the water out if allowed and contains a home made pitot tube in to calculate air velocity thru its own Manometer. Question? to measure the bench flow should I be simply inserting my 1/4 inch (second vert manometer)tube into the side of this runner with the table top test hole open to the atmosphere and then read the Mnometer? Or should I cover the Table top hole except for the manometer tube and control my flow inches with the runner valve, then open the hole mount the carb and test mods