I agree with ChasBean on designing around the pressures.
You can go to the ventilation and infiltration chapter in ASHRAE fundamentals and look into leakage calculations. These leakage calculations can be pretty intense, and not necessarily that accurate. Not because they can't be accurate, but because you can't expect to have such a detailed knowledge of your inputs. Even if you knew exactly how much doors were supposed to be undercut and how sealed the building is supposed to be which should all be detailed in the architect's project description and/or specifications, the final product may or may not be exactly as specified. The uncertainty created by what someone else (i.e. the contractor) feels is "close enough" to specifications is massive when you consider all of the variables. You're going to find the governing equations behind how leakage between spaces and through the envelope (because of wind, because of stack effect, and pressurization) have some non-linearities and overall it is a mess to waste your time with. These are nice things to be aware of, and you should be, but probably not the best route to your solution.
If you don't want to reduce your exhaust rate, then you will have to increase your OA rate. You cannot get around this. 10% or so greater supply than exhaust either by increasing supply or decreasing exhaust, while meeting your requirements for each.
It may change your design some and you'll have to be creative, especially because you need the space to space differential and not just inside to outside. Your VAV for the 5 Pa will have to supply air based on both maintaining pressure, and temperature, which creates issues with overcooling. Reheat on that box is your most simple solution. I worked on a high security overseas project way back when and the 5 Pa space was the emergency room and it was pretty much standard conference room sized. Just run the box to that room off a differential pressure sensor to maintain + 7.5Pa between that room and adjacent spaces and add reheat. For the rest of the building you may overventilate while using inside/outside differential pressure sensors to control your exhaust fan VFDs to a 2.5 Pa setpoint.
