How spine width is calculated
A perfect-bound spine is exactly as thick as the stack of interior pages. Offset and digital printers publish a PPI (pages per inch) per paper stock, so spine = pages / PPI. Amazon KDP publishes per-page thickness instead: white paper and standard colour 0.002252 in, cream 0.0025 in, premium colour 0.002347 in, so spine = pages x thickness. Comix Wellspring's calculator works the same way: 0.0025 in for 60#, 0.0028 in for 70# and 0.002 in for 80# interior paper. Both are the same formula; the tool stores each stock the way its printer states it so the numbers can be checked against the source.
Example: 96 pages on 70 lb uncoated (370 PPI) = 0.2595 in, or 6.59 mm. The same book on KDP white paper is 96 x 0.002252 = 0.2162 in.
Wraparound cover layout
A perfect-bound cover is one piece of art: back cover, spine, front cover, plus bleed on all four outside edges. Width = 2 x trim width + spine + 2 x bleed; height = trim height + 2 x bleed. The template PDF marks trims in cyan and spine folds in magenta. Always confirm the final spine with your printer before setting type on it; paper batches vary and a spine off by a sixteenth of an inch visibly wraps around the corner.
Saddle-stitched books have no spine
Floppies and minis up to about 48 pages are folded and stapled, so there is no printed spine. If that is your format you only need a front and back cover at trim size plus bleed. More in how to calculate comic spine width.
Questions
How do I calculate spine width for a comic?
Divide the interior page count by the paper stock’s pages per inch, or multiply pages by the per-page thickness your printer publishes. 96 pages on 70 lb uncoated (370 PPI) is about 0.26 inches.
Does a saddle-stitched comic have a spine?
No. Stapled books fold flat and have no printed spine; the cover is just front and back at trim size plus bleed.