project/cut on a curved solid

I already figured a work around but would like suggestions of a simple way to create a sketch offset from a curved surface, project that
sketch onto the curved surface and cut into the surface "X" amount and have the result remain at the "X" amount throughout, without the cut being deeper at the high point of the curved part.
Also, I see we can join 2 parts together but can we create 2 parts and use one part as a cutter?
Thanks in advance.
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