what is the rule of thirds?

Pryso, you beat me to it.
I'm not sure, but I believe that Peter Walker of Quad fame might have been one of,if not the first to promote the "rule of thirds".
If at all possible, if one can avoid even multiples from ceiling, floor as well as length and width it might be beneficial.

Yeah, 1/3 or 1/5 or 1/7....
Any distance that does not allow 'equal' different distances. (including the one you cannot change, the floor to speaker/speaker to ceiling.
Try to use odd fractions, and calculate the distances in like inches, for all the dimentions. When different dimensions equal the same distance.. bad. when the different dimensions equal all different lengths that are not multiples of 2 or 4 of each other GOOD.

fwiw the rule of thirds was the best setup for my Maggie 1.6s

The 'Golden Ratio" rule is just as important along these lines of thought. The ratio is 1.6 : 1 which was discovered during the Greek times thousands of years ago.

It can be used for room size propotions. For example if the room height is 8 feet, then the room width should be
1.6 times that or 13 feet. If the room width is 13 feet then the room length should be 1.6 times that or 21 feet. This will eliminate certain frequencies from being accentuated in the room.

If the distance from the front of the room to the speakers is 1 then the distance from the left side of the room to the right speaker is 1.6 times that. Same goes for the the other speaker. That will leave a relative distance of .6 between the side of each wall and the closest speaker.

Under these conditions, the distance between the two speakers is 1 as well as the distance from each speaker to the sweet spot is 1, thus forming an equilateral triangle.

This will tell you not only the relative room proportions, but where everything is positioned in the room.

This application of the golden ratio to audio rooms was first developed by George Cardas. You can check out more on this on the internet. Do a search for the golden ratio for audio uses

Enter 'room mode calculator' in google and enter golden ratio numbers and see that there are few bass nodes of note.

As a starting point for an all-new room, this is a very good way to go. Or, even to produce a low-resonance speaker enclosure.

Phi is one of the 'magic' numbers and has been known for centuries. Artists, architects, audio designers, and most importantly Nature herself have all made good use of this number.