Symmetry plane to selection

Hi.
Add new tool, setup symmetry plane (center) to selection poligon

There is orientation issue if try to set up symmetry plane to selection.

Have you tried the streamline tool, Object/Mesh Pivot? You can use edge snapping to snap to the center edge in your sample.

I want a quick way to align the symmetry plane on selection
This moment workflow
exit edit mode> select Pivot option> enable snapping> element to snapping the pivot
My workflow
Select element > press hotKey

actually in this situation i use object/mesh pivot streamline and then mirror it to object pivot or you can use symmetry plane but you must cycle the symmetry plane first.

my suggestion:
maybe make some streamline tool when the axis its click(left click positive and right click negative)  or use the drag (top/right positive and bot/left negative) its create the miror object base on pivot subobject selection.

You can create a composite tool to move symmetry plane.

I do not use symmetry often, so have just had a quick look to see.

Set manipulator orientation/position to "Selection" and select polygons.

compositie tool:-
Common modeling shortcut tools >> P_Align Pivot to Manipulator (Input string:- True, True, False, False, False)
Common modeling shortcut tools >> Align Mesh Pivot To Object Base
# That will move object/mesh pivot to manipulator
Common modeling shortcut tools >> Set Symmetry Axis(Y)$
# Set symmetry axis to the selection normal/Y direction
Common modeling shortcut tools >> Delete
# Delete current sub_object selection to prevent overlapping polygons
Common modeling shortcut tools >> Symmetry Remove-positive (weld/NoWeld)
# Create/enable symmetry