I believe that the current mapping is actually just fine. Any flat surface in real life will reflect different parts of its environment, and if you must have uniform shading on co-planar surfaces you can simply switch to orthographic view.
However, if you view a larger flat surface, depending on which debugging matcap you use, the surface may appear convex or concave to a varied extent. Although it’s a small issue, any further adjustments could break reflection checkers in more crucial situations such as this one.
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RainerTrummer’s
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