I take my test samples from near the skimmer in the deep end since my returns are in the shallow end. I also wait about 30 mintues after adding chemicals (if liquid or predissolved) and brush the pool to make sure they are mixed. If I add anything dry I brush until it all seems to be dissolved and then wait about 6 hours to overnight before testing. I wait 24 hours before tesing salt, baking soda (for TA) or borax since it seems to take a while for the chemistry to stabilize after adding them. If the chemicals have mixed I don't think it really matters if the pump is on or off.