I use the low-range PO4 colorimeter, which is good up to 3ppm. Above 3ppm, dilute 5:1 with RO or distilled water, then multiply result by 5. I also use the alkalinity (KH) colorimeter. Both are definitely able to provide more precision than the typical hobby-grade reagent tests we use ...if such precision is desired.
Coincidentally, there is a promising discussion, regarding the SW nitrate colorimeter, going on here: NO3 Hanna Colorimeter
Coincidentally, there is a promising discussion, regarding the SW nitrate colorimeter, going on here: NO3 Hanna Colorimeter