What would be the problem with using a slide valve on the outside wall of a flowbench, between the orifice wall and the blower compartment, that just creates a "leak" for adjusting depression?
Do a search I believe there has been a few posts about this topic. Mouse was working on one a few weeks/months ago. I don't know if he ever finished it or not. I believe there are a few other members that have tried to make one. I don't know if they worked or not. It has been a while since I saw a post on them.
Flow Performance 3" PWM valve can be "plugged" into the side of your bench to bleed off pressure to obtain the desired test pressure.
You just plug it into the FP1, tell the FP1 what pressure window to find (eg..9.95 to 10.05"wc) and the FP1 will control the valve to obtain your test pressure. Newest version uses a stronger motor for improved performance. $299.