I’m often required to cut a hole into an expensive item, like a custom made cabinet. One false move and a drawer or door can be ruined. In situations where I need to make a precision cut I always make a template first. I’ll use a piece of scrap wood and when I’m happy with the outcome I’ll trace my template onto the job for a perfect cut. See photos below.

Here I’m using a piece of scrap wood to make a template.
I traced the template to the top of this cabinet transitioning a circular exhaust vent into a necessary (for this job) oval one.