The computer system doesn't just "know" what to set Quota at. Panhellenic runs different Quota scenarios (#'s) within the range through the computer system to see the possible results. For example Panhellenic may plug in the lowest possible range number of Quota = X and analyze those results. They look at how many chapters would make Quota, how many would miss Quota, and how many PNMs would not get a bid at all (but might be eligible for Quota Additions if they maximized their options). Then they run it with Quota = X+1, then X+2, etc.
The final Quota number selected is the one where the most Chapters achieve Quota and the most PNMs receive bids through matching. Then Quota Additions are determined.