I will try to clarify the doubt.
In the example mentioned; The shot data is: 166 yds +1 foot and a headwind of 14 m/h at 45°approx (blowing around 7:30 if we see it on the sphere of a clock).
THE FIRST CONFUSION MAYBE GENERATED when he calls an entire grid at 4.5 cells and assumes, which seems correct, that it measures 25 yards per side when "viewed" from a distance of 100 yds, others will call this 1/2 grid (indeed, he said this).
In his first calculation at 0:38, he performs the calculation to find out what the apparent size of the grid is from this distance; the equation is:
X = (166yds/100yds)*25yds---> X=15yds and this is the "apparent size of the complete grid" (4.5 cells) seen from 166 yds.
Then comes the part where he calculates the drift due to the wind. AND IN 1:12 IS WHERE THE SECOND CONFUSION MYBE IS GENERATED; He says "... now obviously this is about a half wind I would call this about 55% so we know that's about 15 yards ..." He says 15 yds instead of 14 yds (or MPH) which is the speed of the wind (it's just a little mistake on his side); if you look at his calculator he performs the operation 14*0.6=8.4; "... this is about 60% we'll say 55, 60 so we know it's 8.4 yards" which is the lateral drift due to the wind.
Finally at 1:28 he performs the following calculation: if 1 grid is equal to 15 yds, then 8.4 yds will be x's parts of 1 grid which (8.4*1)/15=0.56 grid; then if 1 grid is 4.5 cells, 0.56 grid will be x's cells ---------> (0.56*4.5)/1= 2.42 cells.
All of this can be summed up as:
1- (100/Dist)*25 = A
2-Drift/A = B (Drift, it is the lateral deflection, in yards, that you must calculate, due the wind)
3- B*4.5 = # (number of cells that must be moved)
Video link: https://www.youtube.com/watch?v=udez47XHG_c