Bill Cody. Your reference makes me happy that I avoided that field! The vey last paragraph sums it up well:

"Measurements from various laboratories for phosphorus concentrations less than 50 µg/L
have shown a large variability and have proven to be a challenge. While laboratories follow
standard procedures, some variability in results still exist. Variations above 100 percent are
not uncommon. A new standard procedure for phosphorus analysis is needed to eliminate
this variability.'

And these are tests under certified laboratory conditions, not hobby tests!

50 ug/L is 0.05 ppm, which is commonly quoted as the lowest value that is safe from the process of eutrophication. So the answer to my question seems to be: if you see any phosphorous in your water with any hobby test, it's probably too much.