I think 20ppm of O2 is where you're doing more harm than good.
but testing O2 levels ain't simple.
I run my pure O2 for 60 seconds in a 5 gallon batch, moving the aeration wand/stone around the bottom to distribute the O2 throughout.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10