Good Day Nick 

We can suggest that you add a "Calculated Field" on the Customer Layout Designer in order to achieve this.

Please note that we do not assist with the customization of the invoice layouts/reports.

We do have list of advisers on Sage that can assist with Custom Layout Designer, please click on the link below to find one near you.
https://www.sage.com/en-za/find-a-partner/

You can refer to the below post as well that can help:
https://www.sagecity.com/za/sage-one-accounting-south-africa/f/sage-one-accounting-south-africa-general-discussion/148368/adding-a-calculated-field-onto-a-quote-format/389790#389790

Attached is a Custom Layout Designer guide.

0121.Custom Layout Designer.pdf


Kind Regards

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