Appreciate the kind words! If others could find it useful, I’ll revise the webpage to include more detailed instructions on using the tool and post a separate thread.
The source code is written in JavaScript so it’s visible to anyone who wants to look at the math.
I‘m not a surveyor by any means. The calculated lat/lon points could be more precise. Using it on my property that has 80+ corners, I was able to locate almost all my monuments within 6” of where the tool calculated they would be.
Technical - the tool “should” take into account the curvature of the earth. I tried multiple distance formulas and projecting to a horizontal plane. I wound up going with the Vincenty formula. It got me close enough. Open to criticism and suggestions to make the tool more useful.