bl_info = {
    "name": "Snap Cursor to Normals",
    "blender": (3, 5, 1),
    "category": "3D View",
}
import bpy
import bmesh
from mathutils import Matrix, Vector
from functools import reduce
class View3DSnapCursorNormal(bpy.types.Operator):
  """Snap 3D cursor to faces orientated to average normal"""
  bl_idname = "view3d.snap_cursor_to_normals"
  bl_label = "Cursor to Normals"
  bl_options = {'REGISTER'}
  
  def execute(self, context):
    scene = bpy.context.scene
    obj = bpy.context.edit_object
    if (obj == None): return {'CANCELLED'}
    mat = obj.matrix_world.copy()
    bm  = bmesh.from_edit_mesh(obj.data)
    
    selected_faces = [ face for face in bm.faces if face.select ]
    selected_verts = [ vert for vert in bm.verts if vert.select ]
    
    if (len(selected_faces) > 0):
      normals = [ face.normal for face in selected_faces ]
      
      orientation = reduce(lambda a, b: a + b, normals).normalized();
      orientation = orientation.to_track_quat('Z', 'Y')
      
      position = reduce(lambda a, b: a + b,
                                  map(lambda f: f.calc_center_median(),
                                  selected_faces))
      position = position / len(selected_faces)
  
      mat = mat @ Matrix.LocRotScale(position, orientation, None)
      
      bpy.context.scene.cursor.location = mat.to_translation()
      bpy.context.scene.cursor.rotation_euler = mat.to_euler()
    else:
      return {'CANCELLED'}
      
    return {'FINISHED'}
def menu_func(self, context):
  self.layout.operator(View3DSnapCursorNormal.bl_idname)
def register():
  bpy.utils.register_class(View3DSnapCursorNormal)
  bpy.types.VIEW3D_MT_snap.append(menu_func)
def unregister():
  bpy.utils.unregister_class(View3DSnapCursorNormal)
  bpy.types.VIEW3D_MT_snap.remove(menu_func)
if __name__ == "__main__":
  register()

This is a Blender add-on that adds a function
to snap the 3D Cursor to a selection of faces,
aligning it to their average normal.
This is useful for e.g. flattening a set of faces.
### Installation
In the add-ons section of Blender's preferences window,
click 'Install', give it `cursor_normals.py`, then enable it.
### Usage
A "Cursor to Normals" button is added to the _non-pie_ version
of the (Shift S) Snap menu.
To switch to this menu, return to the preferences window,
find 3D View → Snap under Keymap,
switch the command from `wm.call_menu_pie` to `wm.call_menu`,
and switch the argument from `VIEW3D_MT_snap_pie` to `VIEW3D_mt_snap`.
The Snap menu will now be an ordinary menu, with 
### Compatibility
Only tested with Blender version 3.5.1.
### License
This add-on is licensed under the GNU General Public License, version 3.