Our pillows wouldn’t be the same without pure duck feathers and down inserts that we have used and tested for over two years already.
I found the perfect mix to be 75/25, as it gives you that desired plumpness and comfort while still keeping its shape.
As a rule of the thumb is to use a 2” bigger insert to have a nice fill.
For example a 20”x 20’’ pillow cover will require a 22 x 22” insert.
And any other dimensions, as long as the cover is square.
For a rectangular shape I would recommend to stay true to the size (get the same size insert as the pillow cover is listed), it looks more natural.
*All these recommendations are based on my personal experience working with the great quality inserts, and there the link is provided for your convenience to shop directly https://albanyfoam.com/product/down-and-feather-pillows-25-down-75-feather/