In general, I don't think any Oncologist or Urologist is going to recommend any action until you hit > 0.03 and rising (I think many won't go ahead with treatment until 0.05) There are many studies on prognositic accuracy of Ultrasensative PSA results. 0.01 is generally considered a "coin toss" point. Other studies think that .006 is useful but there are too many other variables I think to accurately predict the reliability at that low of a level. In a study the cohort is going to have varying pre and post treatment factors including pre-treatment PSA, genetics, lifestyle factors, etc. My University of Washington, known for their Urology clinic and cancer expertise, uses a test that considers undectable to be < 0.02. Which makes sense as some studies have shown men > 0.01 and < 0.03 that never had recurrence. You don't want to overtreat. Salvage treatment used to be done with two consecutive reading > 0.2! The > 0.03 cutoff now gives roughly 2 years more advance warning compared to the 0.2 cutoff giving the salvage treatment a much higher chance of success.

This is a good article:
https://prostatecancerinfolink.net/2015/09/12/johns-hopkins-ultrasensitive-psa-after-surgery-predicts-biochemical-relapse/
I would guess you would have gotten salvage treatment by now if you hit a 0.1 (not 0.01)