http://www.atlantadivorceattorneyblog.com/divorce/child_support/parenting_time_deviation/
The post notes that ". . The statute, however, gives no guidelines for what the deviation should be."
And here's a case where a parent was denied a deviation:
707 S.E.2d 344 (2011)
WILLIS v. WILLIS; and vice versa.
Nos. S10F1357, S10F1358.Supreme Court of Georgia.January 24, 2011.
Reconsideration Denied February 28, 2011