I’d second Gaspode’s insistence on the importance of story. It’s the story that gives rise to the structure, not the other way around.
I’m leery of formulas: “Do this by page x, do that by page y, etc.”
Great scripts have been written that violate the structural paradigms of Field, McKee and other gurus. And there are backrooms and trash cans full of paint-by-number screenplays which follow the gurus’ dictates to the letter.
Format, on the other hand, defines the screenplay in much the same way that the number of lines, the meter, and rhyme scheme defines the sonnet. Most working screenwriters stick to the standard format for practical reasons. Standardization allows everyone, from actors to studio heads to costumers to grips to get the information they need out of the script with a minimum of confusion. The format allows for a good rough estimate of running time - 1 page = 1 minute of screen time.