Don't fear the foam!
Oh, wait, wrong subject.
Get a glass of water. Put your stone in that and adjust the pressure until you see bubbles. Back the pressure til the bubbles go away and stop. And then adjust forward again until they are very light. Make a note of this pressure and use it to oxygenate your wort.
Basically, you want to see bubbles but you don't want to churn the water. O2 that reaches the top is not going into solution. It's going into atmosphere.
Ideally, if possible, you could get a flowmeter and find the point where you have positive flow with little to no sign of bubbles (if possible) indicating that the O2 flow is going into solution thus maximizing DO and minimizing waste.