hi, making a first time cushion for my sofa of down....i have new material to make the cover to put the down cushion in when i layout the material to cut...how do i allow for the " fluff " of the down insert ...i want the bolster to be 3.5" thick...but i want the " fluff " taller in the middle of the seat... how do i cut the top and bottom to allow for the fluff so the down doesn't get smashed with the usual style cushion - like a regular piece of foam ? is there a formula to cut the material properly to allow for this arch ?

Thanks for your help !

Trisha