I have an 18' AG round pool. When we bought the pool we got an 18' winter cover. The cover barely fits over the pool, especially with the air pillow the cover only goes about 6" over the top of the pool. I have read in the past you should always go up a size when buying a winter cover. What size do you recommend for my pool? Is the next size round pool 21'? I was thinking since the pool is so small and I always keep on top of draining the cover that I might not need the air pillow, it's a pain to keep centered in the pool.