One note/question on making new cushions: I'm not a professional, but I believe that typically the cushion covers are made to the proper size and "stuffed" slightly with the foam pieces. That is, the foam pieces are slightly oversized so that the covers will be nice and tight.
So I'm thinking that if one is tracing old foam pieces, they're good to go; but if using the cushion covers for patterns, you'd want to add a bit of size to the foam
Do I have this correct?
More or less. When I made my covers, I traced the shape to be 3/4" extra on all sides of the cushion foam, so that it can be hemmed ove by 3/8" I made the gussets so that they were about 1/4" inch narrower than the foam thickness when stitched onto the top and bottom, and the covers have to be stuffed. I used heavy YKK zipper stock buried in the gussets. After a few days the covers had assumed the shape of the foam perfectly.
Be sure to make all the straight sides as straight as possible when hemming.