I'd revisit the first few posts and just accept your current levels and dosing of everything ...if you're happy with your tank. 40 ppm NO3 won't hurt, although it's much higher than many of us like it to be. If you are into fooling around with targeted parameters (like me and many others, here), then spend the time working the permutations. If you want hands-off, stay where you are and collect the ooohs and ahhhs.
At this point, while still fine-tuning, I only use NO3 as a guide for how well my fish are dosing ammonia and how well I'm dosing urea. My NO3 target for this purpose is about 5 ppm and, I agree with @PlantedRich
that the Salifert kit is far beyond the API capability and well-worth the extra cost.