% of total of the X first rows

Hi everyone,

I have a visualization than updates daily with the most sold products of the previous day + the % of total per each product.

I'm displaying only the 20 most sold products (rank by quantity)  because the list can be sometimes very long, and I would like to add a new chart indicating the % of total of those 20 products (everyday they are different), but I don't know how to do the calculation.

Can someone help me with this?

Thank you very much!