18" sounds like a lot to me, too. I guess it depends on how much rain and snow you typically get over the winter. Maybe compromise and do 12" since you had a problem last year with not having it drained low enough.

For my pool, I have always done about 6 inches. But, I recently bought a new cover. Unlike my old cover which let no water in, this one is a tightly woven mesh which will let water drain through it into the pool. So, I will probably go a little lower than in past years.