Optimal Surround locations

 For Dolby Atmos, it's 1 foot above and 6" in front of ear, according to a UK magazine:

https://www.audioadvice.com/blogs/expert-advice/home-theater-speaker-layout-options

That's a bit less than 90 degrees.

Google AI suggested 90-110 degrees from center from some other source for both 5.1 and 7.1 "surround."  That's what official Dolby Atmos website shows for 7.1.  But for 5.1 Dolby Atmos shows 110-120 degrees in their diagram but the picture shows 90 degree direct side placement.

Crutchfield says "to the side pointed at listener" for 5.1, without giving any specific number of degrees, but the picture shows the 5.1 surrounds directly to the side, showing the Dolby picture (but not their diagram).

Discussion at AudioScienceReview quoted ITU-R international standard which specifies 100-120 degrees.  They also clipped a bit from Floyd Toole's book showing arguing as you get closer to 90 and 120 the effect diminishes.  (So, in this telling, 90 and 120 are equivalent, so it seems to me the range should be 100-110 or 90-120.)

But Floyd Toole doesn't actually show 100 or 110, just 90 and 120.  And the 90 and 120 graphs (which are not entirely comparable, as the 90 degree one has no center channel) have virtually identical scores!  

Toole also shows a surround setup with speakers at 30 degrees and 60 degrees, and that seems to have the best score of all!  But I haven't seen any discussion of that.  Yamaha famously had a 7.1 or 9.1 system with wide front Presence speakers as one option.  I have one of those receivers still in my Kitchen which I use mostly for stereo but will do some semblance of 5.1 and 7.1 either direct from Oppo or synthesized by receiver with my own manual fine tuning.  I have never done wide front 'presences speakers, though my front speakers are 30 degrees (I myself prefer 45 degrees as in my living room system for the widest possible stereo, which also means the most information).

Here's what leading Harman scientist (under Floyd Toole) Dr Sean Olive said about surround layout (in 2018):

 For example, if you’re setting up a stereo system around a TV or project screen, you’ll want to try and get the right and left front speakers flanking the screen at ±30 degrees angles relative to your seat in front of the screen to ensure a best-class sound experience. For surround sound the left and right side and rear speakers should be at angles of ±60-100 degrees, and ±135-150 degrees, respectively. After setting up the surround sound, it’s critical that consumers leverage the test sequence in the receiver, which will help ensure that every speaker is set at the proper level and time arrival.

Perahps Harman was not on the same page as Dolby and others.  The Harman ideas were based on their own objective psychoacoustic research in their laboratory in California.  (However, I need to get more detail on Dr. Olive's reasonings. than provided here.)  Perhaps it's in Toole's book, which I think I have.

But also note that Dr Olive is implicitly assuming both side and rear speakers!  So it's not really answering the question here (what's best for 5.1) but 7.1, and for 7.1 nearly all authorities either include 90 degrees in their rage (often showing it in pictures) or specify it exactly.  But for a room of listeners, only those lucky enough to be on axis with the side speakers will get 90 degrees, and those in the back will get...well 60 degrees if they're lucky (a point seldom discussed).

Random commenter at AVSForum suggested 90 degrees for 7.1, and somewhere in between that and rear for 5.1. 

I could easily do 90 degrees in my living room for listeners on the back couch which is right on the back wall.

Some don't think that is right even for 7.1.

Then what are you supposed to do for couch on back wall?  Every other side position is further front.