I recommend shocking if you use your spa 3 or more times a week.
An ounce sounds like a lot. To shock 400 gallons, 3-4 TEASP00Ns should be adequate, not sure if that measures one ounce or not. It is important when you shock to leave the cover off for an hour to allow off-gasing to occur.
Stuart, are you saying that you add MPS each time you use the spa or are you adding a small amount of dichlor? Adding a small amount of dichlor after using is not by definition shocking but sanitizing.
HTM