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I don’t measure heat, mostly use hot water from the tap, heat until you can bend easily, usually too hot to hold in your hand. If you need to, reheat.

You are over-thinking it. Hot enough to bend....

"What you received looks ok to me and is certainly workable. all horn will eventually warp on the shelf, unless bound.”

Another good reason to use a proper fitting wedge, as opposed to a spacer.
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Yup, normal.

As said cut to rough thickness and shape first, less material to work. Then use steam, hot water, or heat gun, they all work. Once heated it is very pliable, I clamp in a wooden...