The website provides answers to FAQ, including 11. How is the pull force of each magnet determined? which explains that "All of the pull force values we specify have been tested in our laboratory."
FAQ #12 explains futher that "Most other online calculators are based on theoretical formulas, which are notoriously inaccurate, especially for very large or very small sizes. Our fanatical engineers have worked long and hard in the laboratory developing our online calculators that are VERY accurate based on thousands of test cases."
If your interest is practical, you could create a table of values from the website, covering the range which you intend to use, and interpolate between them. If your interest is theoretical, take a look at Force between two point dipoles. Note that a magnet induces a mirror-image magnet in a conductor, so the force between the magnet and a conducting plate at distance $d$ is the same as that between two magnets at distance $2d$.