Do you know the ppm of O2 at pitching? Another variable...
And yes, blowoff is a good sign......There is fermentation taking place.
On fermentation taking place, it is a good thing.
On PPM of o2, I performed my usual treatment, 45 seconds of pure o2 through a stone for ales.
I saw an o2 meter on the website of the University Surplus store for $15 and thought about taking a chance. I'm sure it would need a new probe, but I haven't looked into how much a replacement would cost.