Thank you for the advice everyone. Going to speak with Hanna and see if I can get some free reagents. I’ll start testing same time every day and averaging over a week to get more consistent results. I’ll also try the older ones and see what the deal is. It was fluctuating from 0-20/25ppm.
In addition:
1. Make sure there aren't bubbles in the tube when taking your reference measurement
2. Make sure there aren't any smudges/oils/fingerprints on the cuvette for either measurement
3. Try to keep the cuvette in the same position during both measurements -- I like to position it so I can read the writing on the cuvette before I insert it into the checker.
4. Make sure you're getting all the reagent out -- cut off 2 sides and push the sides of the packet in to create a funnel before dumping it into the cuvette.