The YouTube video from The Funky Little Chair has this great fabric cutting guide. Just fill in the dimensions of your chair and it helps you to calculate out the dimensions of the pieces to cut out. This is a cut sheet for a box cushion with welt.
Then I used the information on the cut list to decide how the pieces will be laid out, and to determine how much fabric is needed.
I want to cut the welting on the bias, which allows more "give" when bending the cording around the corners. But cutting a 45 inch bias does leave a lot of wasted fabric. A YouTube video by Fabric Farms Interiors suggests using a "60 inch bias", where the piece is cut at a slight diagonal. There's less wasted fabric, but you still get the benefit of a little bit of bias cut.
