I have a 20 x 40, with a built in pool cover. I usually add a tarp cover over the pool cover to minimize leaf and debris stain. There is approx. 5' concrete apron in the shallow end where the pool cover slides over the steps. Including this additional area, the dimensions would be 20x50. Would a 30x55 cover work? Or do I need to go larger? Obviously when the water level is dropped during winterizing, the cover sags/drops requiring a larger cover than the actual pool size. Please advise. Mils?