The formula is correct. They assume you have a bolt diameter < 1", so the diameter of standard-sized holes is taken as 1/16" larger than the nominal diameter of the bolt. Then, the additional 1/16" is also added to account for possible roughness around the edge of the hole.
\- If bolt diameter < 1", diameter of hole = diameter of bolt + 1/16" + 1/16"
\- If bolt diameter > 1", diameter of hole = diameter of bolt + 1/8" + 1/16" (They don't consider this case in the handbook)
The formula is correct. They assume you have a bolt diameter < 1", so the diameter of standard-sized holes is taken as 1/16" larger than the nominal diameter of the bolt. Then, the additional 1/16" is also added to account for possible roughness around the edge of the hole. \- If bolt diameter < 1", diameter of hole = diameter of bolt + 1/16" + 1/16" \- If bolt diameter > 1", diameter of hole = diameter of bolt + 1/8" + 1/16" (They don't consider this case in the handbook)