Find reference plane, for surface flatness analysis

We have got a number of points (considered as z axes) on a surface. For
example the given are the (x,y,z) coordinates of points on the surface:
(1,1,2)(1,2,4)(1,3,5)(1,4,3)(2,1,2)(2,2,3)(2,3,1)(2,4,6)(3,1,3)(3,2,6)(3,3,2)(3,4,1)

Now we have to find a reference plane from these z coordinates. The
problem we are having is that how to find a reference plane for these
points on the surface so that we could define the tolerance range using
this reference plane. Read about something like least square fit, is it
possible to calculate with that method? Is it possible to find a
surface using Integration where the volume becomes zero?

Thanking you

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