Four thicknesses, not one
A pared cover is thick in the middle and thin at the edges, and the useful way to think about it is as four separate measurements rather than as one number. Over the board it is the overall pared thickness — 0.9 mm at the values in the form, sitting on 0.098 in of board. In the joint it is thinner still, because the leather folds twice in a gap under a third of an inch wide, and two thicknesses of 0.6 mm is already 0.047 in of leather in a joint that has to hinge.
Where the turn-in stops, five eighths of an inch in from the edge, the leather is on the slope between the edge thickness and full thickness. At 0.3 mm at the edge and 0.9 mm at an inch in, that lands at 0.68 mm — and that is the ridge the pastedown has to bridge, running all the way round the inside of the board.
The corner is a stack, not a thickness
At a mitred corner the two turn-ins meet on a cut line, so there is one layer of edge leather there and the corner is 0.3 mm. Lap one turn-in over the other instead and there are two, at 0.6 mm. A library corner, where the leather is pleated rather than cut, puts three layers in the same place at 0.9 mm — the same as the whole pared cover in the field, concentrated at the one point that gets knocked.
Which is why the edge thickness matters more than any other single figure. Unpared at 1.2 mm, the same lapped corner would be 2.4 mm and the library corner 3.6 mm. The paring cuts the corner stack by three quarters, and the corner is the only place where every thousandth is multiplied.
The bevel is a slope you cannot see
Going from 0.9 mm to 0.3 mm over an inch is a drop of 0.0236 in across 1 in. That is an angle of 1.35 degrees, or a slope of one in 42. Nobody can see that on a piece of leather and almost nobody can feel it directly — what you feel is the leather draping differently, and what you see is light raking across the flesh side.
It also explains why paring is slow and why a dull knife is useless at it. A slope that shallow is made by removing a shaving of a few thousandths at a time over an inch of width, repeatedly, and any knife that tears rather than cuts leaves a dig that shows through the finished cover.
How much leather ends up on the floor
The blank at the defaults is 14.946 by 10.5 in, which is 156.9 square inches, and the pared band round its edge is 47 of those. The blank as it arrives holds 7.41 cubic inches of leather; taking the field from 1.2 mm to 0.9 and running the band down to a 0.3 edge removes 2.41 of them, which is 32 percent. That is worth knowing when the skin is priced by the square foot, because roughly a third of the cost of the leather in a fine binding is literally swept up.
None of which says anything about how thin your cover should be. That depends on the skin, on where in the skin it was cut, on the board and the joint, and on how the leather behaves when it is wet with paste. This page counts; it does not advise.
Questions people ask
How thin should I pare leather for a book cover?
This page does not answer that and no page honestly can. It depends on the skin, on which part of the skin the piece came from, on the board caliper, on the joint width and on how the leather behaves when it is damp with paste. What the page does is take the thicknesses you decide on and show you what they produce at the edge, at the turn-in and at the corner, so the consequences of a choice are visible before the knife goes on.
Why does the corner need to be thinner than everywhere else?
Because it is the only place where layers stack. A mitred corner has one layer of edge leather; a lapped one has two; a library corner has three. At 0.3 mm at the edge those are 0.3, 0.6 and 0.9 mm. Leave the edge unpared at 1.2 mm and the same three become 1.2, 2.4 and 3.6 — a lump at the exact spot a book gets knocked when it is pulled off a shelf.
What angle is a paring bevel?
At the values in the form, 1.35 degrees — 0.9 mm down to 0.3 mm across a one inch band, which is a slope of one in 42. That is far too shallow to judge by eye, which is why the knife is held nearly flat and the work is checked against raking light rather than by looking at the edge.
How much leather does paring remove?
At the defaults, about a third of the leather in the blank by volume — 2.41 cubic inches out of 7.41. Most of that comes from taking the whole field down from 1.2 mm to 0.9, not from the band round the edge, because the field is seven tenths of the area. If the skin is priced by the square foot, a quarter of what you paid for it ends up as shavings.
Should the pared band be wider than the turn-in?
The page flags it when it is not, because a band narrower than the turn-in puts the step back up to full thickness inside the turn-in rather than under the pastedown, leaving a ridge a short way in from the board edge. Whether that matters, and what to do about it, is a decision about the binding. The arithmetic only shows you where the step lands.