When sizing power vent CFM where does the 0.7 come from? Where does the 300 come from for power vent intake requirements?

Here is some information about sizing power fans.

The factor 0.7 in the power vent calculations comes from two things: achieving 10-12 air changes per hour (ACH) and an attic assumed to be 8 feet tall at the ridge. In fact, working out the numbers, 10 ACH is a factor of 0.66 and 12 ACH is a factor of 0.8. The factor of 300 for intake (dividing CFM by 300) is to prevent an air speed through the intake openings capable of lifting and carrying debris. A 300-ft/min air speed is a low limit, which will prevent drafting. For example, room air diffusers for air conditioners are often sized to provide an air speed of 700 ft./min this provides good room coverage without giving the sensation to occupants that there is a draft.