I have a 2-speed Jacuzzi pump with a cartridge filter on an 18" A/G pool. The directions say to clean the cartridge when the pressure gauge gets to the "clean" marking on the dial. There is no dang "clean" marking on the gauge! What pressure reading should I have before I open'er up and change the cartridge? (I have two) How often do people clean their cartridges in a normal season? The bather load in our pool has been minimum as the weather in Ohio hasn't been cooperating! Figures...put up a pool...and can't get in the thing!!