the logic behind Converting spend to exposure metrics

Hello, I would like to know the logic behing converting spend variables to expousers like impressions and click rate. what's your idea about computing Gross Rating Points (GRPs) or Target Audience Rating Points (TARPs) for TV and radio and "readership" for Print.is it possible to compute GRP based on what [The Incremental Reach and Cost Efficiency of Online Video Ads over TV Ads](https://research.google/pubs/pub40426/) said and then use that GRP for generating target variable?

"to describe the relationship between unique reach and Gross Rating Points(GRPs) or impressions of a TV campaign:" where R stands for reach and G stands for GRPs.

R= a− (b/G+(b/a))