Build on the current jacket template, not the trim size alone.
The trimmed page size is only one input. A hardcover case usually projects beyond the text block, the spine reflects the bound book thickness, and the jacket needs folds and flaps that wrap around the boards. Bleed, hinge or joint areas and safe zones are then added according to the selected construction. A rectangle made from page width plus a guessed spine will not describe that object reliably.
Finish the interior pagination and confirm the text paper before locking the jacket. The same page count on a bulkier stock can produce another spine width, and a change in case style can move folds even when the pages do not change. Use the printer’s generated template for the exact job; measurements published by a self-publishing platform describe that platform, not every casebound book.
Flaps are useful reading space, but they remain folded components.
A synopsis, contributor note, series list or illustration can sit on the flaps, provided it clears folds and trim. Keep type away from the fold itself and allow for the natural curve where the jacket turns around the fore-edge. A centred block should be centred in the safe flap area, not simply in the geometric panel shown before production allowances.
Long flap text deserves the same editorial care as a back cover. Read it at final size, check line endings and remove information that will date quickly unless the jacket is versioned accordingly. If the cover carries a barcode or price block, identify who supplies it and protect its required clear area. For barcodes and price blocks, apply the contrast and clear-area checks in the 2D barcode guidance.
Wrap-around artwork must survive several folds without a false promise of alignment.
A landscape, colour field or repeating pattern can travel from back flap to front flap, but every fold interrupts the way it is seen. Keep faces, titles, narrow rules and other alignment-sensitive details off the spine and fore-edge folds unless the design deliberately accepts movement. A narrow border around each panel makes small folding differences more noticeable.
Extend intended backgrounds through the bleed and wrap areas shown on the template. Do not enlarge the whole jacket PDF merely to create missing bleed; that also moves the spine, fold positions and type. The bleed, trim and safe-area checks keep background extension separate from live content near a finished edge.
Proof the jacket flat, folded and on a representative case.
A flat PDF is the right place to check copy, image quality, panel order, fold guides and colour separations. A full-size folded mock-up answers another set of questions: whether the spine reads straight, whether flap text starts too close to the turn, and whether the front image feels balanced once wrapped around the board.
For colour, surface and finish, ask what the physical sample represents. A prototype made by another process may be excellent for fit but only approximate for colour or coating. Record the approved jacket filename and template version together. Assign each approval to the question it can genuinely answer using the proof comparison.