Add product weight in invoice lines.

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- Database: [MySQL 5.7.27]
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Hi all,
I need to add a calculated weight of etch product line in my invoices.
Etch line should look like the attached image:

The calculation of the weight is simple: product net price/3= calculated weight
The “product net price” is already in the invoice from so this is looks like an easy task.
After some research I realized that the best method to do so is to use extra fields.
Using the tutorials : “Extrafields” and “Add Extrafields on PDF Models” I manage to create a new PDF template for my invoices, add a “Line complementary attribute” and include it in my ne PDF template with no issues.
My knowledge stops at the point of creating the needed formula for the “complementary attribute” to fetch the line unit net price, divide it by three and print it in my PDF invoice.
Any help will be greatly appreciated.
Thank you, Dennis