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I use Cheap Mirror Glass, its always flat enough. For 30x30cm Printbed there are Mirrors called "LOTS" at Ikea, they do it well.

I prefer to print directly on mirror, especially petg. Clean the mirror only with warm water and soap. Drying it with some paper-towels and then never ever touch the surface again. See https://www.youtube.com/watch?v=Kj4Xz3DwGSQ

Sometimes i use a Mirror with some Lokbuild apllied on it, with pla i do not need to heat the bed.

To check flatness: Put a steel-ruler standing on the surface and check against some light. You will see the light come through under the ruler, if its not flat. Just like this: https://www.youtube.com/watch?v=xUX2LSzOr3o