Thats why when I test its 3 days after a spray and/or feed. I think that's a good time because the plants had time to ingest some good food and the spray has had time to completely dry up. Im sure theres residue but its minimal at best. Maybe ill use a clean cloth to kinda dust off the leaves I pick next time...we'll see I reckon
My thinking is, the leaf has a fair amount of surface area to collect the sprays. You roll it up and squeeze out a small drop of juice that passes through all that dust/foliar residue/other crud. I seems like the tiny amount of liquid could easily be contaminated. If so, some/all might add to the brix reading, but would never reduce the readings. A quick rinse and dry could give a more accurate reading.
I'm going to test both ways to see if it makes any difference.