Me!
When I started tracking this stuff, I did my due diligence. The formula is the one Deadline and other trade mags use. Other websites corroborated such calculations. When Deadline, e.g., publishes an article with a list of the top ten profitable movies for the year... it's this formula they use.
And the formula exists because studios won't release actual figures. So, it's just a 'rule of thumb' based on data that is sometimes definitively known about a few movies, which is then extrapolated to all movies.
So, here is the reasoning...
In addition to the budget to make the movie, a studio spends *on average* another 50% of that budgetary number on advertising and other administrative fees. I presume the cost of distribution is a big part of that.
And the Box Office receipts are split between the studios and the theaters. Now, when a movie is first released, the studio's cut is more than 50%... maybe more like 60%. But over time as the movie plays, the theater gets a bigger and bigger cut. But that's domestically. Internationally, the theaters get the larger cut. So, as a rule of thumb *on average* the receipts are just considered equally split.
And again, this rule of thumb is because we don't know the real figures. We don't know if a studio decided to spend a minimum on advertising because they didn't think the movie would do well, or splurged on advertising for a blockbuster.
That's why I keep emphasizing it's a "rule of thumb" and "on average." And that a final bottom line of anywhere from a profit of $10M to a loss of $10M should be considered 'break even' because of the vagaries of the black box of studio accounting.
The benefit of the Rule of Thumb is the ability to compare one movie's finances to another with an equal hand and make the comparisons more equitable and not based on 'feeling,' such as when people say, "Surely it made a huge profit because me and all my friends saw it and liked it!!!