Those are primarily used by electricians to create a knockout in a breaker panel and it would only work if you removed the sink.

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I'm a general contractor with 35 years of experience in construction.

OP stated that it’s an undermount sink which, by definition, means the sink does not rest on the countertop because it is underneath the countertop and not on top of it.

This is the best answer so far.

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Follow the same process of marking the wall and ceiling every few feet to make sure the crown doesn't roll on you and viola.

For inside corners, one end of the crown is cut square and butts against the wall. The other end is cut mitered for an inside corner first and then coped. The process for measuring remains the same.

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But, unless the crown is too big to cut "standing up" like shown in the picture linked above there's no reason to cut it flat. The picture linked above shows how to cut crown "upside down and backwards" for lack of a better way to describe it.