I feel your pain! I have a differently printed card (same colors, tho) and mine is between 10 and 20 routinely. I am always wondering if I should put the tube smack up against the white card, or hold it slightly off so that light goes through the tube. I've decided to do the latter over time, partly because I like the results better, and it's more intuitive to me.
That said, I have learned from others on this forum a couple of things that I'll pass on to you:
1. Nitrate is plant fertilizer. The more plant bulk you get in your tank, the happier the plants are and the less you need to stress about NO3 levels.
2. Plant consumption of NO3 doesn't mean you don't have to do regular water changes. The only real way to make sure your levels stay good is to regularly (weekly) do a major (50% or better) water change.
Tanks a bunch!
My current 120 tank journal:
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