Except as been told to you before your assuming that its a uniform 60/40 split across the board for all studios on all movies on all theaters (and specifically in all countries internationally) for its entire run, and that is not the case. For example, again something you've been told, Disney famously had a more generous 65/45 splits in years prior for the first week or two of release. Who knows what it is now, that is not something publicized. And that percentage drops off the longer a movie is in theaters. So even assuming its 60/40, its not 60/40 for the movies entire run.
@BrianLo did the work to back check and actually match up the calculations to the actual yearly Deadline breakdown on the movies and Disney financials. And if he is saying you are under reporting it for the most part, I would believe him.
This is why using 2.5x is just easier for our purposes, no need to quibble over falling percentages and such.