I just installed a sump (20 gal long mod. myself) I have a refgum. which is (13' L X 12' W X11' H my skimmer section is 10' X 12' X 7' and my pump section is 4' X 12' X 7') I have 20 lbs. of live sand which makes my sand bed 3 1/2" deep and 1 pound of live rock in refgum. My over flow box is rated at 300 gph and my pump is rated at 700 gph I use a ball joint to regulate the return flow. My question is everyday I have to add water to the sump or the water level in the pump section goes below my marked point. Could I be losing water to evaporation that quickly or is there some other reason for the water level to be so low (1" or more in the pump section)?

Unless you see water on the floor or around the inside of the stand, it's just normal evaporation. You notice the evaporation much sooner since it's just the small pump return section of the sump that drops.

My return section in my sump is 16x12x15tall and before installing my auto-topoff, my return section used to drop 2-3inches per day due to evaporation.

Yup, auto topoff is a life saver. I lose almost 2.5 gallons per day and I am soon going to add another fan or two to get more evaporation so I can drip more kalkwasser. The sps corals are growing and requiring more and more calcium and alk.