The falling curve
An oil finish is not a stack of coats, it is a filling operation that gets harder as it goes. The first application meets dry wood and disappears. The second meets wood that already has oil in the outer fibres and takes less. By the fourth or fifth the surface is taking almost nothing and what you are doing is spreading a thin film and wiping it back off, which is a different job from feeding the wood.
Model that as a ratio and the arithmetic becomes simple. If each application takes 60 percent of what the one before took, and the first took 0.06 fluid ounces per square foot, the series runs 0.06, 0.036, 0.0216, 0.013, 0.0078. Set a threshold at 0.01 and the fourth application is the last one that clears it. Over 40 square feet those four add up to 5.22 fluid ounces, and the whole infinite series, if you never stopped, would only ever reach 6 fluid ounces. That last fact is the useful one: past a certain point, more applications are not putting meaningfully more oil into the wood.
Getting your own numbers
Both figures come off a sample and neither can come off a label. Oil a board of known area, weigh or measure what left the tin, and you have the first application rate. Do it again the next day and the ratio between the two is your falloff. Ten minutes of measuring gives you a number that holds for that species and that surface preparation for good.
Expect wide variation. Open-grained wood takes more than close-grained. A surface sanded to 180 takes more than the same wood sanded to 320, because the finer surface has less to hold. End grain can take several times what a face takes, which is why a table with breadboard ends and exposed end grain on the underside is not one uptake rate but three.
The part that is not arithmetic
Oil-soaked rags heat as they cure. Not sometimes, and not only with one product: it is what the curing reaction does, and a wadded pile holds the heat in. Shops and houses have burned down this way, and it remains the most common serious accident in finishing because the rag is the last thing anyone is thinking about at the end of a session. The product safety data sheet tells you what it wants done with them, and that is the authority on it.
Where this sits
For film-forming finishes the arithmetic is thickness rather than uptake, which is the film build calculator. Quantities by product type across a job, including oils alongside varnishes, are in the finish coverage calculator, and the days a schedule takes are in the finishing schedule calculator. How finely the surface was sanded changes uptake directly, which is the grit progression calculator.
Questions people ask
How do I measure how much oil the first application takes?
Weigh the tin before and after, or decant a measured amount into a small cup and see what is left. Do it on a board of known area that you have prepared exactly the way the real work will be prepared, because sanding grit changes the answer. Divide the volume used by the area and that is your rate. It is worth doing once per species you use regularly, and worth writing on the inside of a cabinet door, because you will want it again.
What falloff figure should I use?
Yours, from two measured applications on the same sample: divide the second by the first. Anything from a steep drop on close-grained wood to a shallow one on thirsty open-grained stock is plausible, and the page defaults to 0.6 only so the form has something in it. The figure matters most for the total, since a shallow falloff means many applications each taking a real amount, and a steep one means the job is effectively over after three.
Does more applications mean a better finish?
Past saturation it means a thin surface film rather than more oil in the wood, which is a different thing and not automatically an improvement. The page shows the sum of the whole infinite series next to what your schedule actually reaches, so you can see how much is left on the table. When the schedule has already captured 95 percent of what the wood will ever take, another pass is buying appearance and evenness rather than penetration.
Why is my end grain still drinking after four applications?
Because end grain is open tube ends rather than closed cell walls, and it absorbs along the grain far faster than a face absorbs across it. In practice that means the ends of a board are on a different curve entirely, and running this page twice, once for the faces and once for the end grain area, is more honest than averaging. It is also why end grain goes darker with the same colouring product, which is a colour problem rather than a quantity one.
Can I use this for hardwax oils and oil-varnish blends?
The uptake model fits anything that soaks in and gets wiped back, so it fits many of them, but the further a product moves toward building a film on the surface the less the falling-uptake picture describes what is happening. A product that leaves a measurable film is better handled as thickness, which is the film build calculator. If you are not sure which yours is, the data sheet will say whether it is meant to be wiped back to bare or left to build.