As soon as the water level gets to the lowest point of the overflow it sucks air and breaks the siphon and stops... Do you have a internal or external???? Shoot a pic if ya can....
If you wanted to do the math is easy take lxwxh and divide it by 231 =x now take x and divide by the heigth say it's 2.1 now that means for every inch of water in your tank there is 2.1 gallons, if the lowest part of your overflow is 2" down your gonna have 4.2 gal wash back.... For the sump same thing let's say it measures 16x12x12 so that equals 2,304 divide by 231 that's 9.97 gallons now divide that 9.97 by 12 the height so for every inch of water in the sump you have .83 gallons.... Make sense now???? If you ran your sump at fifty percent you would have enough for 5 gallons wash back..... Hth please feel free to ask if not lol... I know I can be confusing sometimes if not all the time...:tongue: