How to do a plane fitting of a mesh from 3D scanning?

How to do a plane fitting of a mesh from 3D scanning?
I am doing different projects with 3D scanning, but have a challenge to make proper plane fitting of the mesh surface to horizontal plane surface. When I do the 3D scanning with my 3D scanner it will not always be in correct level or direction with the horizontal plane. I usually use the floor surface from the 3D scan to determine how much off the mesh is from horizontal zero plane. I have search for this and tried different software, but not been able to find anything useful yet for this.

Have some of you in this group experience or some tips how to solve this challenge? Software?

Thank you!

... I'm either adding calibration points (normed spheres) to the part before scanning it or scan a part of the table to get the coordinates/fitting points to adjust the plane to ...

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Viktor
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FreeCAD is foremost a solid CAD modeler but it does have some mesh tools. There's an align utility (in the Edit menu) to align two selected objects with1 to three corresponding points. I used it just last week to align the bottom face of a scanned mesh to the XY plane. I created a cube to get a planar face to pick on. Once my mesh was aligned to the top face of the cube I deleted the cube and exported the mesh back to STL.

Now this might be moot if your mesh does not have a planar face...