I am a fan of feeding more personally because i am a huge proponent of overfeeding, but also understand why people would rather just dose the nutrients. I think the most important thing is to get the nutrients in there whichever way works for you.
If I were you I would make a fresh 5g batch of saltwater and then test the N/P to get a baseline(which is hopefully zero or nearly so). Then I would dose your N/P in the bucket up to what should be a significant and easily detectable amount and test again. If the numbers come out close to what they should based off your dosing this can rule out bad kits or user testing error as well as conform your N/P additives are not the issue.
If I were you I would make a fresh 5g batch of saltwater and then test the N/P to get a baseline(which is hopefully zero or nearly so). Then I would dose your N/P in the bucket up to what should be a significant and easily detectable amount and test again. If the numbers come out close to what they should based off your dosing this can rule out bad kits or user testing error as well as conform your N/P additives are not the issue.