Hi Andy, that dial is the setting for the rinse aid.The rinse aid only really helps the drying.If you are getting a poor wash, I would check the spray arms first,if the holes are blocked,they will not rotate so give a poor wash.You may get a poor rinse which leaves silica (looks like sand) on the upturned bases of your mugs.
Hope this helps. All the best.
FOXY
September 2008