Can I enter a spine width manually in inches or millimeters?
Yes. Choose Inches or Millimeters under Spine Width and enter the exact value supplied by your printer. This is useful for paper stocks other than KDP cream or white, or for another printing company with its own spine formula.
Do not estimate the value from a different edition. Ask the printer for the spine width based on your final trim, binding, paper, and page count, then remake the print cover with that measurement.