I would have thort, if you had a good pump, you would have to set the relief to control your peak vacuum.
I have 5" at idle & 10" at WOT with a Moroso 4 vane pump. As mentioned above, you might have a leak so maybe you need to test your system.
I do know my pump is worth 0.12 to my ET. when it had a fail we looked at where the power went & found the pump drive was faulty. Once fixed the missing ET returned.