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Vinny, elucidate me... Why does five inches more matter so much? Is it a problem to lift your leg up or something? (Seriously, I'm not joking, I don't know.) We had a spa recessed into a deck so that about only four inches showed, but that turned out to be a pain for service and the like. I'd not do that again.
...a sales person suggested not to sink it all the way into the deck - it would make it harder to get out and 18" is half way up the tub...