Two effects pulling opposite ways
Wet leather stretches onto a form and then shrinks as it dries, and the two are not the same number and do not cancel. The stretch acts on the girth, so it makes the blank smaller than the surface it covers. The shrink acts on the finished part, so it makes the part smaller than the form. A page that folded both into one percentage would hide the fact that they apply to different dimensions.
On the default form — 7 by 4 by 1-1/4 deep — the girth along the length is 9.43 inches. Four percent of stretch means a 9.07 inch blank does the job, and the grip margin adds two inches on top of that. Meanwhile three percent of shrink takes the dry part from 7 inches to 6.79.
The corner is where the length goes
Measure the form square and you get length plus twice the depth: 7 plus 2.5, so 9.5 inches. Work the actual path with a quarter inch radius and 6 oz leather at a half neutral axis and you get 9.43. The difference is a sixteenth of an inch, which sounds trivial until you are trimming a formed part and find the margin uneven on one side.
The radius matters more as it grows. Take the same form with a 3/4 inch radius and the girth drops to 9.00 — exactly half an inch shorter than the square measurement. Rounding a corner shortens the path the leather has to take, which is the opposite of what most people guess.
One point of shrink
Three percent shrink on a 7 inch form gives 6.79 inches. Four percent gives 6.72. That 0.07 inch difference is the whole gap between a moulded part that drops over what it is meant to fit and one that has to be persuaded. It is also well within the range that soak time, water temperature and drying speed will move you. This is why the field asks for a figure from your own test piece and why the page prints the sensitivity: nobody can hand you a shrinkage number for your leather.
Where the model stops
It works two girths independently, one along the length and one across the width. A real pull couples them — at the corners of a deep form the leather has to travel further in both directions at once and cannot, so it either gathers into folds or thins out. Shallow forms with generous radii behave close to the arithmetic. Deep forms with tight corners do not, and no amount of adjusting the neutral axis factor will make them. The page is a starting blank and a test piece is the only thing that settles it.
Questions people ask
How much bigger should a wet moulding blank be than the form?
It is not a percentage, it is a path length. The leather has to travel across the top, round each corner and down each side, and then you add whatever margin gets held under the clamp. On a 7 by 4 by 1-1/4 form with a quarter inch radius the girth along the length is 9.43 inches, and with a one inch grip margin each side the blank is 11.43 inches long before any stretch is allowed for.
How much does leather shrink after wet forming?
That belongs to your leather, your soak and your drying, and the page treats it as a field rather than stating a figure. What it will show you is how much the answer moves: on a 7 inch form, going from 3 percent to 4 percent shrink costs another 0.07 inches on the finished part. Mark two points on a test piece, form it, dry it fully and measure — that is your number and nobody else can supply it.
Why does a bigger corner radius need a shorter blank?
Because a rounded corner is a shorter path than a square one. Going square round a corner means travelling the full radius out and the full radius down. Going round an arc means travelling a quarter circle, which is shorter than the two straight legs it replaces. On the default form, taking the radius from 1/4 to 3/4 inch shortens the girth by 0.43 of an inch.
What is the neutral axis factor for leather?
The page defaults it to half because that is the tidy assumption, and says plainly that it is a placeholder rather than a measured property. It is where in the thickness the leather neither stretches nor compresses as it bends round the corner. If your formed parts come out consistently long or short against the arithmetic, form one, measure the actual girth, and solve backwards for the factor that fits your leather.
Does this work for deep forms with sharp corners?
Less well, and the page says so rather than pretending otherwise. It works the two girths separately, and a real three dimensional pull couples them — at the corner of a deep form the leather needs to travel further in both directions at once, so it gathers or thins. Shallow forms with generous radii track the arithmetic closely. Deep ones need a test piece.