You want to add nutes first and adjust it if it needs to be, when you water you just add the pH down to get it to where it needs to be, after you water you test your pH run off.
So you test the pH of the water with and without nutes and test the pH after you use the water/nutes on the plant
So you test the pH of the water with and without nutes and test the pH after you use the water/nutes on the plant