First of all; It depends on the average to begin with.
Secondly; It depends which course is played.
WGT takes a 9 hole score and doubles it to give an 18 hole score. If you play Kiawah-par 72, a 1 under, 35 becomes a 70, -2.
If you play Bethpage-par 70, a 1 under 34 becomes a 68.
So for example: If the average is 68 to begin with and you play Bethpage at 1 under for 9 holes(converted to 68 for 18 holes) the ave. stays the same. If it`s Kiawah, 35x2=70, then the ave, goes up. Hope this helps.