I have revised a pattern that uses 2 1/2 inch jelly rolls. Instead I wanted to use a (42 piece) 10" layer cake I just brought. There was only 2 or 3 prints that I liked, the rest was ugly as far as I was concerned. Will never buy another layer cake. I recommend cutting your own 10 inch squares. The jelly rolls you can kind of see what you are getting before buying but only makes a 6 1/2 square.
So using my layer cake squares, I cut my center 3 inch sq, then added 2 (3" by 3 1/2" long)sewn on each side of the 3 inch sq. On each side of this I cut 2 ( 3 1/2 wide by 10")to go on either side of the center strip and then trimmed the ends off. Now I snowball the top left and bottom right with 3 1/2 white squares with line drawn through the centers, sewn and cut off the corner leaving 1/4 inch seam. This makes an almost 9 inch square. Put these rows together so that you end up with the snowballed ends making a square too.
Hope I haven't confused you too much. I will add an inner 2 1/2 inch border and 5 1/2 inch outer border. You may want to try this quilt.
Sandy$