[bhaveshattarde]

If I have a component what is requiring a flatness of 20 microns so while designing a fixture what should be the flatness we should consider while resting it on rest pads.?

[Jim Dawson]

There are so many variables here that it impossible to give a definitive answer. But I would expect the fixture to be 100% flat. Place the fixture and then machine the supporting pads in-place. The best you can get is your machine tolerance. The part flatness may depend on your machining strategy.