1.  Enable the option to show Margins on Sales Orders:

       Sales --> Configuration --> Settings  

2.  Add the field via the User Interface:

      Settings --> Technical --> Database Structure --> Fields

      Create

The code for the Compute section is:

for record in self:
    if record['amount_total'] > 0: 
        record['x_margin_percentage'] = record['margin'] / record['amount_total']

3. Add the field to the User Interface, by creating a view that inherits and overrides the Odoo Sales Order Form view that adds the margin field:

      Settings --> Technical --> User Interface --> Views

      Create

The XML for the Architecture field is:

<field name="margin" position="after">    
    <field name="x_margin_percentage">
</field>

Finished Customization: