I have read Ben talking about blowing sand out during backwashing, and that you can throttle an oversized pump down with a valve, which I have done. My question is how can someone tell if they need to add more sand to their filter? When you fill your sand filter with new sand, you put in "so many" pounds, whatever it takes, but if you don't note the level on the inside of the filter, you have no way of knowing how much you are losing. My filter's owners manual does not describe a certain level or measurement from the inside of the filter. This question is certainly filter specific, and if I need to I can post my filter's info, but is there some description out there for an internal measurement of sand? Thanks.