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shim.
Although I have never measured the slope, mine has similar slope to yours. Like Chas said, this is not a matter that would affect the operability or reliability of the spa. Rather it is a cosmetic issue. 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. I like that. What can I say? With a similar situation, I elected not to shim mine and don't even notice it anymore.Bill