I created grids (A1, A2....,B1,B2,...etc.) in print layout (main properties-grids).I also use atlas. I display many points and I need to calculate location of every point within a page of atlas and a grid. It is simple to calculate location of a point within a page atlas, because atlas is driven by a grid layer. I don´t know how to simply calculate location of a point within a grid.
On a picture below black line is atlas grid frame (overlapping), red line are grids created at prin layout

How can be it done? 
[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/1aWbt.jpg