Using refractometer to determine effectiveness of foliar spray?

I came across this use of a refractometer on plant leaves in my search for the efficacy of using Alaska Fish Fertilizer + Kelp as a foliar spray.

Item #9. states: “Foliar programs can be determined with a refractometer. Several one square metre patches can be sprayed with different foliars at representative rates. The foliar formula that produced the biggest brix increase within a 40 min period is the foliar that will be most productive in your crop. Use your refractometer to remove the guess work!”

Do you think this makes sense? Or is the refractometer simply measuring the opacity of plant juice amended with fish juice?

Your opinion?

Most of the time I have seen people using it for measuring the sugar levels on the fruits.

Yes I use my refractometer for fruit. Was wondering if anyone ever used it for leaf sap. The statement about evaluating foliar feeding via refractometer sounded intriguing!