How is KI calculated?

Can anyone tell me how the KI value in SAP PA is calculated. From various SAP docs the following is given: "This is the quality indicator of the models generated by Automated Analytics. It corresponds to the proportion of information contained in the target variable that the explanatory variables are able to explain."

But how is it calculated? What is the equation that leads to a result of KI = x%?

Many thanks

Ria