If this measurement is unknown: Use a piece of duct tape, or a pencil, to mark the exact water level on the faceplate of the skimmer box, or if necessary, on the pool wall near the stairs or ladder. Wait 24hrs, re-mark water level and use a measuring device, like a ruler, to calculate the rate of loss (inches/24hr). Label your markings with date and time. If this result is inconclusive, leave original markings and repeat this process as needed. Please leave all labeled markings on the pool for the diver to observe.