Choose Home tab→Sheet Metal group→Mid-Surface 
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Tip:
In Solid Edge Simulation, the Mid-Surface command is located on the Simulation tab→Geometry group.
On the Mid-Surface command bar, specify the offset side.
The side you specify is highlighted in the graphic window.
On the command bar, specify the offset ratio.
Click the Preview button.
Finish the feature.
Tip:
The offset value must be between 0 and 1.0.
By default, a mid-surface updates when the model geometry is changed; you cannot edit a mid-surface independently of the sheet metal part. You can remove this associativity permanently by selecting the mid-surface in PathFinder, and then selecting the Break command on its shortcut menu.
You can edit the inputs used to create the associative mid-surface by first selecting it in the graphics window, and then selecting its edit definition handle, for example, Mid-Surface 1.