Use a chemical test kit that you like, or at least a test strip. All those pens and stuff are either junk, or are too expensive to ever pay for themselves. I have yet to find a pen or meter that’s worth a darn, and I say that having been a Lab Tech since 1991.

The problem you’ll have there is, getting the plants at the correct height to be the right distance from the light. It’s a constant struggle for me, with my bad back.