Why the middle sticks out
A saddle-stitched booklet is a stack of sheets folded together, and each sheet has to wrap round everything inside it. The cover fold sits furthest out; the centre fold sits at the middle of the stack. Every leaf is the same length from its own fold, so the leaf whose fold is furthest in reaches furthest out at the other end. That is why the middle pages of a magazine stick past the cover before it is trimmed.
The amount is the caliper of one sheet times the number of sheets between the cover and the centre. Sixty-four pages is sixteen folded sheets, so fifteen gaps, and at 0.004 inches a sheet the innermost leaf projects 0.060 inches — a sixteenth of an inch, or 1.5 mm.
Trimming flush is what does the damage
The guillotine takes the fore edge off in one cut, so the inner pages lose 0.060 more than the cover does. A layout drawn with a 0.375 inch fore-edge margin ends up with 0.315 in the middle of the book and 0.375 at the front. On a booklet with a rule down the outside edge or a folio hard against it, that difference is obvious as soon as you flick through it, and on a thicker booklet it can take the folio off entirely.
The correction moves the problem, it does not remove it
Shingling slides the content on each leaf towards the fold by the amount that leaf will lose. The cover moves nothing, the centre spread moves the full 0.060, and everything in between moves in even steps of 0.004 a sheet. After trimming, every fore-edge margin is 0.375 again.
What it costs is the gutter. The centre spread's content is now 0.060 closer to the fold, so a 0.5 inch gutter becomes 0.44 there. That is usually a good trade, because a gutter has slack and a trimmed fore edge does not, but it is a trade rather than a fix — and if the design puts a rule or a page number near the fold, it moves too.
Measure a dummy rather than trusting the caliper
The arithmetic above uses flat caliper, and a fold is not flat. Real folds have a radius, bulky uncoated stock springs open, and a heavy plus cover pushes the block more than its own thickness suggests. Every one of those makes the real push-out bigger, never smaller, so the calculated figure is a floor.
The fix takes ten minutes: fold the real page count of the real stock, stand it on a bench, and measure how far the middle leaf projects past the cover. Put that number into the page in place of the caliper and the shift table becomes yours rather than generic. This is the same reason printers keep a stack of unprinted dummies around.
When the booklet is too thick to stitch
Push-out doubles as the page count doubles. The default stock at 32 pages pushes out 0.028 inches; at 64 it is 0.060; at 96 it is 0.092. Past somewhere in that range the spine of a stitched booklet stops sitting flat, the fold on the outer sheets starts to crack, and the fore edge trims into a visible wedge. Where exactly that happens depends on the stock, the folder and the stitcher, and it is a judgement for the printer rather than a number this page can supply.
Questions people ask
What is creep in a saddle-stitched booklet?
It is the amount the inner pages project past the cover once the sheets are folded together, also called push-out. Each sheet wraps round the ones inside it, so the innermost leaf ends up furthest out at the fore edge. Trimming flush then takes more margin off the inner pages than off the cover, and the difference is what has to be corrected in the layout.
How much creep does a 64-page booklet have?
On stock with a 0.004 inch caliper, 0.060 inches — sixteen folded sheets, fifteen gaps between the cover fold and the centre fold, 0.004 each. That is about a sixteenth of an inch, or 1.5 mm. Double the page count and you roughly double it; move to a bulkier stock and it climbs again in direct proportion to the caliper.
Which direction do I shift the pages?
Towards the fold, by the amount that leaf will lose. The cover moves nothing and the centre spread moves the full push-out, with even steps in between. Both pages of a spread move inward, so the left page goes right and the right page goes left, and anything crossing the gutter has to be built to still meet afterwards.
Does shingling cost me anything?
It costs gutter. The content on the inner pages is now closer to the fold by the full push-out, so a half inch gutter becomes 0.44 inches at the centre of a 64-page booklet. That is normally the better place to spend it, because a gutter has slack and a trimmed fore edge has none, but it is a trade rather than a free correction.
Should I use the caliper or measure a dummy?
Measure a dummy if you can. Flat caliper ignores the radius of the fold, the spring of a bulky stock and the way a heavy cover pushes the block, and every one of those makes the real push-out larger than the calculation. Fold the real page count of the real paper, measure how far the middle leaf projects, and enter that instead — the page will work the effective caliper back out of it.