When I design a quilt using a panel, I decide what size to make my blocks, then measure the panel and whatever it needs to make it an equal size for matching my blocks to is how I determine the inner border size. If blocks are going to be 12" and panel is 33 wide, then I add 1.5" border on each long side so 3 blocks wide at top and bottom will line up evenly. I do the same for length of panel. Be sure to include seam allowance.