A bit hard to quantify "regulator on full blast". Is that an actual regulator, or a needle valve? And are you using an air stone, or an open wand?
With a good (.5 micron stainless steel) air stone, if your starter is a quart of around 40 point wort and you pushed in a liter or so of O2, if you assume a 50% absorption rate you'd be in the 20ppm range which is about as high as you want to go (40ppm gets into the toxic-to-yeast range).
With an open wand the bubbles are comparatively humungous and absorption is a lot lower. I wouldn't even begin to guess how many liters of O2 at "full blast" you'd have to push to hit 20ppm with that setup.
For reference, the best O2 concentration you can achieve using atmospheric air is 8 ppm...
Cheers!