I don't own a nitrate test kit, I tested for more than a year and never saw it read anything but non-detectable on Salifert so I never bothered to order another one.
I haven't tested Phosphate in a long time.
To get a consistant reading I recommend that you only test in the morning before you have done any feeding. If you have feed the tank already for the day you tend to get higher results and hard to track overtime.
My target is .06-.08ppm which seems to be the sweet spot. Anything more than .1ppm tend to start seeing algae issues and normally just adjust feeding amounts to compensate ( I still feed 3 time per day ).
Using Hanna HI736 Phosphorus ULR:
Phosphurus reading 16ppb .
Converting to Phosphate PPM ( 16X3.066/1000 ) is .049ppm
I have not used any GFO or other phosphate removers in more than a year. Only exports are Algae Turf Scrubber and 1 Cup of Rox .8 Carbon in BRS Reactor changed out weekly. Occasional once a month will run a filter sock and siphon the shallow sandbed.
Just for grins:
I am going to run 2 tablespoons ( 1/8 cup ) of GFO mixed with 1 Cup Rox .8 Carbon in the Reactor changed out weekly and track it for a while see what kind of results.