Per your description above I'm assuming the wort in the BK has been cooled before transfer through O2 injection? I strongly suggest that it is or your absorption will be minimal at best. I few factors to consider with in-line injection that can effect O2 absorption and desired levels in finished wort are Temperature (cooler wort will absorb more O2), Gravity (bigger beers require more O2), Flow of wort across injection stone, Size of injection stone, Volume of O2 (LPM) etc. To calculate O2 absorption without measurement is a tricky game. Kinda like guessing the gravity of wort without a hydrometer, Good thing is with O2 its not as critical. In fact a little too much O2 is better than not enough.
That said I would set your flow to around 2GPM and your O2 at .5 - 1.5 LPM depending on the gravity and temp. My O2 stone is in a sight glass so I can see the wort moving by and the bubbles from the injection. The sight glass is especially helpful when starting the injection as sometimes you have to turn the flow up to get it going before you reduce it to the proper flow. It is also important if possible to fill your fermenter from the bottom. If your using a bucket or carboy put you line at the bottom. This allows the new oxygenated wort coming in a better chance of holding the O2 in and if it does come out it passes through the rest of the wort on its way up. Adding pure O2 is more about preservation and control for me as I want as little stress as possible on my yeast and I like the control factor for repeatability.
