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shim.
Although I have never measured the slope, mine has similar slope to yours. Â Like Chas said, this is not a matter that would affect the operability or reliability of the spa. Â Rather it is a cosmetic issue. Â I personally think that having the whole base of the spa in touch with the pad is bound to put less stress on the frame. Â I like that. Â What can I say? Â With a similar situation, I elected not to shim mine and don't even notice it anymore.Bill