When in 1950 Paul Fitts was working with measuring the time it took to move an object from one location to another, he wasn’t thinking about computer graphics.
Computer graphics as we know it today, with high resolution bitmapped images was yet to be invented. He was looking at moving parts on a production line, to make the process faster. But his equation can and is applied when designing computer interfaces, including those for graphics software like Pencil2d.
A video made by the University of Nottingham explains his equation and how to apply it to the design of graphics software today. The following link allows you to view it.
https://youtu.be/E3gS9tjACwU?si=tIdvk6UvYllqpPBp
I hope this puts your understanding of using Pencil2d into perspective!