You can't adjust the opening pressure, it's regulated by the spring stiffness, regardless of how much you tighten them up. Spring ratio is linear & it's the spring ultimately "deciding" when to open the flapper in "boost-off" operation.

It would appear to make a difference when you do a mityvac test (since the volume of air is so tiny), but it's wrong. If overtightened - you're essentially just restricting the arm motion (thus, opening time & max flow through the flapper if it can't open completely anymore).