The cleaner is independent of the cell so that setting will not matter to the cell at all. However, the cell is tied to the filter pump and that is the only setting that really matters as well as the %. If the filter pump is on, the cell will go on for the first part of the cycle.

With 18,000 gallons, 50% setting and 8 hrs run time, you will be adding about 1.7 ppm per day which should be OK depending on you CYA. Check it after a day or two and see how it goes.