a bit of free advice...
you haven't explained why over using the chemical is problematic, to you.if they use more chemical you make more money, right?
To the buyer, using more costs money directly, and indirectly because of potential waste and pollution control implication. So if you want them to use less, the solution might be to suggest that somewhere up the decision chain, higher than the operator, a boss needs to be responsible for how much of that chemical is being used and how much over use might be costing the company. The boss can then create a system by which operators have to explain why they used more of the chemical than the paper tests indicate were really necessary.