How is Related % Calculated?

Related % in the Keyword Magic Tool is based on the URLs being displayed in the top 20 results for the keywords you are analyzing. This metric compares the main searched keyword in Semrush's first two pages of results (what you searched in the top search bar) to the related keyword's first two pages of results. Depending on how these URLs relate, you will receive your Related % metric. 

For example, look at what happens when we search the seed word "buy books" in the Related report.

Compared to the related keywords "book buy books" or "book buy" shown in the results, the related % is 70 and 65. This means that 70% of the URLs that are displayed in the top 20 organic results for "buy books" are also displayed in the top 20 organic results for "book buy books" and  65% of the URLs displayed for "book buy" also displayed for "buy books".