If you Google how far to set an extension ladder away from a wall you see OSHA's flat recommendation of a flat 1 to 4 ratio for the distance the bottom of an extension ladder should be from the wall, regardless of how tall the ladder is.
That is, for every four feet of height pull the ladder bottom out one foot. This gives a universal 75 degree tilt.
But is this always a good idea or even safe? Probably not!
Extension ladder "kick back" is a far more common cause of falls than you're pushing off backwards from the top of a ladder and the whole thing falling straight over.
In fact, on YouTube I couldn't find even one video of a backward fall happening, despite many dozens' of "ladder fails." The overwhelming cause was kickback, the ladder sliding out backwards from underneath because of losing it's grip at the bottom.
Go to Google search for "ladder falls" and watch the videos.
The non-crazy people (who didn't put the ladder on top of milk crates or the like) had the ladder too far from the wall and the whole thing slipped out from under them. But in several of these cases it appears to be within the OSHA 1 to 4 ratio.
And here's where the 1 to 4 ratio gets ridiculous. If the ladder is extended 24 feet that means the recommendation is the bottom be six feet from the wall. There is no way you are going to be able to shift your center of gravity back six feet when you're on top of that! But kickout cold happen if the surface is concrete, or wet, or anything less than prime for traction.
The one time I got kickback and fell it was because I followed the OSHA guidelines and the ladder slid out backwards on a clean concrete surface.
I was using a 20 foot extension ladder on my job cleaning windows on a tall second story with an extension pole. The top of the ladder was probably18 feet. The bottom was around 4.5 feet out and on a totally flat concrete walkway that led to the front door. It was early morning in West Palm Beach, Florida and about 100% humidity with the concrete slightly damp from overnight condensation. But no visible water. And the concrete was maybe two years old, perfectly clean, and in good condition. Without warning suddenly the ladder slid out backwards. Thankfully I wasn't hurt, but from where my feet were I fell straight down 13 feet onto the concrete with a squeegee in one hand and a towel in the other, landing on my feet. Another worker came over and said it was a miracle (which I believe!).
So, 3.5 feet would have been plenty in that set up. There's no way I could lean back that far and pull the whole ladder over backwards.
And if you are 30 feet up, do you really need 7.5 feet? No way! Your center of gravity won't be within 4 feet of that even if you tried to pull the ladder over. But I'm not putting a 30 foot ladder that far back and risking a slide out.