Average Pressure Differential Calculation

If you have a constant exhaust system serving an enclosed space with a self closing door than your min, max and average differential calculation will be the same value.
If you have a variable exhaust and/or VAV (variable air volume) box providing make up air to the space then you would have a different min, max and average differential pressure.

Assume the door is closed.

An ASHRAE equation for Differential Pressure (∆P) in PA is:
Q=165.5*A*√∆P

Here is an example scenario:
3 foot wide closed door with a 0.5" undercut
A=0.125 ft
∆P=5
Solve for Q (Air Flow in CFM)
Q=165.5*0.125*√5 = 46.3 CFM

In order to ensure a room has a negative 5 PA differential pressure to an adjacent space you need to have roughly 50 CFM total exhaust (Exhaust CFM - Supply CFM). Area of the room is irrelevant for this equation but for LEED and local building codes you may also need to ensure that you have at least 0.5 CFM/SF exhaust.

What if there is no undercut in the doors? How should I represent it on the template?

Victor,
The template just wants you to check off a box that you did a design calculation and submit a drawing highlighting which rooms have chemicals/hazardous gases.

If you don't have an undercut in the door the air has to come from somewhere if the space isn't air tight. Could be spaces in the door, cracks in the ceiling grid, etc.

Dylan, some of our reviewers also ask for detailed calculations/charts with cfm and pressurization for every room -- not just checking a box.

There isn't a spot to upload those calculations specifically. But if they do ask for those calculations or you find a spot to upload them, just follow the formula above.

Dylan,
Thanks for posting that equation. Which ASHRAE standard is it from?

Margaret,
See ASHRAE Applications Chapter 52 on Fire and Smoke Management.