You’re not entirely wrong to be cautious. 0.05 mm flatness on a 200 × 200 mm plate is achievable, but it’s tight for a standard 3-axis mill. You’re talking extra passes, slower feeds, and possibly hand scraping or surface grinding if the machine isn’t perfectly tuned – as is quite often the case. Old school manual scraping is great.

For 6061‑T6 at that thickness, a flatness of 0.1 mm is usually safe and inexpensive on a standard milling setup. Grinding or lapping can bring you down to 0.05 mm if you really need it, but it will increase cycle time and scrap risk. If your sensor mounting is sensitive to tilt or warping, you can specify 0.08–0.1 mm and perform a few hand checks or surface-finish checks. Otherwise, 0.05 mm flatness is probably overkill and will cost more than it’s worth.