Implementation shortfall

Does anyone know of a simple way to calculate it and the components? I cannot get my head around it and am continously getting stumped.

watch these videos

  1. https://www.youtube.com/watch?v=O0G3NecQldA

  2. https://www.youtube.com/watch?v=BU6M-y_tv0U

I know I definitely watched the first one for the concepts to sink in. This was by Bruce Chadwick if I am not mistaken.