Duct Design for Positive and Negative Pressure

[titanum456]

I am currently doing a simple calculation for my duct sizing.
I am referring to the ROUND INDUSTRIAL DUCT CONSTRUCTION STANDARDS by SMACNA.
In that guide, it mentioned about duct design for negative pressure and design for positive pressure.
How am I able to know these values ?

[Brian_UK]

Surely the Standards tell you what you need to know?

You ask about values, values of what?