Fitts’ Law.

This scientific law predicts that the time required to rapidly move to a target area is a function of the ratio between the distance to the target and the width of the target.

Fitts's lawWikipedia
Fitts’ Law: Tracking users’ clicksInteraction Design Foundation
Glossary: Fitts’ LawUsability First
Fitts's Law: The Importance of Size and Distance in UI DesignInteraction Design FoundationJune 12th, 2019
An Interactive Visualisation of Fitts's Law with JavaScript and D3Simon WallnerNovember 7th, 2012
Applying Fitts’ Law To Mobile Interface DesigEnvato Tuts+May 9th, 2012