when in doubt, always go to the source.
From npcwomen.org, a snippet from the update document dated October 2015:
With RFM, we have seen chapters stabilize and on most campuses have seen chapters increase in size overall. NPC adopted new total policies in October 2015 to allow chapters to achieve parity as quickly as possible. Total is now to be automatically adjusted every regular academic term. This is to be done no later than 72 hours following bid distribution in the academic term that primary recruitment is held and within one week (no more than seven days) from the start of the academic term(s) in which primary recruitment is not held.
College Panhellenics can use one of the following methods to determine total:
1. Median chapter size
2. Average chapter size
3. Largest chapter size (recommended for College Panhellenics with three or fewer chapters)
Adjusting total every regular academic term (i.e., fall and spring terms for semester schools or fall, winter and spring terms for quarter schools) allows for the most accurate reflection of true Panhellenic size.
Take the time to read the entire document (five pages total, but not all text) because it will give you a much better picture of what's being discussed. There's information on upperclass quota, for example.