I just did a deep cleaning of my filter using a hose. I believe I have lost a good bit of sand in the 11 seasons of operation without ever adding sand before. I have a Hayward Pro-Series S244T that is supposed to hold 300 lbs of sand. Problem is, I see no need to remove the old sand and add back 300 lbs. I just want to add enough more to the proper amount, but I don't know how deep the proper amount would be. The sand surface is currently about 12" below the top of the standpipe in the middle of the filter for reference. I estimate it is about to the middle of the tank, which definitely seems low to me.
How far below the top of the filter should the sand bed be for this filter?