Hi Trevor,
I'm sorry about your diagnosis. Different peoples' prostate cancer produce different levels of PSA, so you can't compare your PSA with anyone else's. (About 15% of prostate cancers produce no extra PSA at all.)
What you can do, to a first approximation, is compare your own readings to each other. This means that the activity of your cancer is only 1% (1.9 / 189) of what it was, which is great news. Cancer cells, by definition, can mutate further, and may do so to form cells which produce more or less PSA, so you can't completely rely on a linear relationship between PSA and cancer cell activity.