The joint allowance is charged per leaf, not per panel
Take a seven inch leaf and shoot a sixteenth off each edge before it meets its neighbour. You have not lost a sixteenth, you have lost an eighth, and an eighth of seven inches is 1.8 percent of every leaf you bought. Halve the trim to a thirty-second and it drops to 0.9 percent. Take an eighth off each edge of a four inch leaf instead and the same job eats 6.3 percent. That share does not change when the panel gets bigger, because it belongs to the leaf rather than to the work.
The part that catches people is the rounding. Leaves across a face is a ceiling, so the last leaf is nearly always mostly offcut, and whether the offcut is a quarter inch or six inches is pure luck of the arithmetic. The page prints it, because that offcut is the thing you can trade against: a slightly narrower panel, a slightly wider substrate for the neighbouring door, and the leaf count drops by one.
Balance is a construction decision, not a finishing one
The convention in cabinet veneering is to mirror the layup about the core. Face veneer on one side, the same species and thickness on the other; crossband under one face, crossband under the other. Each side of the panel then takes up and gives off moisture at a similar rate, and the two are pulling against each other rather than one pulling against nothing.
The reason this is easy to get wrong is that an unbalanced panel looks perfect the day it comes out of the press. It is a seasonal problem, which means it shows up as a curved door six months later in a room whose humidity has moved. The wood movement calculator works out how far a solid board travels over the same humidity swing, which is the comparison that makes the case for veneer in the first place.
Thickness in thousandths, because inches are useless here
Sliced veneer and sawn veneer differ by a factor of four or more, and the difference between a face that can be scraped and one that will be scraped through is a few thousandths. Calipers, not a rule. The page adds the stack in thousandths for that reason and gives the finished panel in decimal inches, so you can check it against the rabbet, the groove or the hinge cup it has to fit.
The other reason to know the finished number before you press is that veneer thickness comes off the substrate you buy, not off the opening. A three quarter inch substrate with 22 thousandths a side finishes at 0.794 inches, which does not fit a three quarter inch dado.
Where this sits
Clamping and pressure for the same layup is the glue and clamp calculator, and if the press is a bag rather than a row of clamps, the pump side of it is the vacuum press pump down calculator. Cutting the substrate out of sheet goods is the sheet nesting yield calculator or the cut list optimizer. For solid stock rather than sheet, the board foot calculator. Once the panel is together, finishing it is the film build calculator, and the grain on an open pored veneer is the grain filler pore volume calculator.
Questions people ask
Why does the leaf count round up so aggressively?
Because a leaf is indivisible across its width. If the panel needs 3.05 leaves you buy four, and the fourth contributes a strip. There is no arrangement that splits the difference, which is why the offcut on the last leaf is printed as its own line: it is the number you negotiate with. Shaving three quarters of an inch off a panel width, or accepting a slightly wider stile, can drop a leaf off every door in a run.
Should I count the crossband as a leaf?
Only if you are cutting it from the same flitch, which is unusual. Crossbanding is normally a cheaper species, often a different thickness, and it runs across the panel rather than along it, so its piece count comes from the panel length rather than the panel width. The page treats it as another veneered layer for the piece count and the glue lines, which is right for the arithmetic. If you buy it separately, run the page a second time with the crossband dimensions in the leaf fields.
What glue coverage should I enter?
Whatever your product data sheet says, or better, whatever you measured. Coat a panel of known area, weigh or measure what came out of the container, and divide. That figure includes your roller, your spreading habit and your substrate, none of which a published range knows about. This page has no coverage figure of its own and does nothing but repeat the one you type, which is why the output line says as entered.
Does the yield percentage tell me if I bought badly?
Not on its own. A low yield on a wide panel made from narrow leaves is arithmetic, not waste, and a flitch is often bought for its figure rather than its width. What the number is good for is comparing two options in front of you: the same panel from seven inch leaves against nine inch leaves, or a trim allowance of a sixteenth against a thirty-second. Run it twice and the difference is real, because everything else is held still.
Can I veneer over an old finish or an old veneer?
That is a substrate question and the honest answer is that nobody can settle it from a description. What can be said is what to do first: on anything made before about 1980, the finish and the filler under it are a testing question, because sanding or stripping a surface that contains lead makes dust of it. And if the piece has any real value, the original veneer and the original finish are most of that value, so it goes to a conservator before it goes to a sander. New work over a clean substrate is the case this page is for.