I subscribe to the equilateral triangle set up as the starting point as well. Sometimes that's not practicable, based on your room, furniture, rear walls, etc.

So based on that, and with the risk of stating the obvious, here are a few tips...

Use a recording of a vocal that is known to be well anchored / mic'd to put the singer in the middle of the speakers.

Play around with towing the speakers in and out to see what effect it has on making the head of the singer appear smaller / tighter, or pulling it apart. You may find the degree of the speaker tows are not completely even to achieve the best effect. This can be due to room acoustics or differences between how acute your hearing is with your left vs. right ear (That may sound strange, but as an example, if you've ever hunted without hearing protection, the ear on your shooting side may be less acute.)

Once you've established a good center image, play around with a few recordings that have bigger soundstages (orchestral pieces, ambient / techno music, or just a good Pink Floyd album), and see if you have a good balance between center fill and soundstage width. Ideally, you should be able to close your eyes and point to sounds that seem to be coming from outside the boundaries of the left and right speakers. If you're not getting this, try towing back out, but not so much that you get the dreaded "hole in the middle."

Similarly, if you have the wiggle room, play around with placement relative to the rear wall and see how it does or does not crush the depth of the soundstage and affect bass response. One common mistake I see is that people put their speakers too close to the wall.

Repeat this process a few times using different recordings and types of music. In the end, you'll reach a set-up that represents a "best compromise" i.e. a commmon denominator of placement that strikes a balance between center fill and soundstage width.