What's the correct monitor height for large monitors?

Conventional ergonomics guides suggest aligning the top of the monitor to where the operator looks at straight on. It seems doubtful that that still applies to today's 24" and 30" and larger displays.

What was the reasoning behind that rule? What's the correct way to position a huge display according to current research?

Would anyone have a reference to research confirming the "2/3 up" rule?

Actually, the rule of thumb is to align your eyes (straight ahead) to about 3/4 up the height of viewable area of the screen. That is, about 1/4 of the screen is above eye-level, the rest is below and this should apply to any reasonable screen size.

Beyond that rule of thumb, the bottom of the screen should not be more than 60 degrees below the straight ahead horizontal viewing angle.

I'll discuss the specific recommendations below, but let me start with the basis for them.

Do the recommendations need to be updated?

The question in this thread was recently re-asked (and marked as a duplicate). The reason was that the information here, and elsewhere, is old ("Has the science changed?"; is the underlying research still current?). Even in this question, the applicability of the research is questioned due to the growing size of monitors ("It seems doubtful that that still applies to today's 24" and 30" and larger displays"). These concerns are understandable given that the OSHA advice cited in one answer shows what looks like 14" CRTs from the early 1980s.

Basis for Current Recommendations

The OSHA recommendations were based on extensive research focused on human physiology. Humans are a few inches taller, on average, than we were centuries ago (thought to be mainly due to diet and medicine), but we have not evolved in the last few decades to adapt to growing monitors. The research behind workspace ergonomics still applies.

How well do the guidelines apply to the enormous monitors of today?

You can answer that with basic trigonometry. The OSHA guidelines discuss an optimum where the screen occupies a visual angle of roughly 30 degrees (top of the screen slightly below straight ahead and the center of the screen down 15 to 20 degrees from straight ahead). The rule of thumb for monitor distance is arm's length (about 28-30"). That gives you a total screen height of roughly 16", equivalent to a 32" monitor in a 16:9 format.

The OSHA guidelines discuss an upper limit where the entire screen fits in a 60 degree visual angle (which would require a little head movement to view top to bottom). A screen that size would be viewed from farther away, and the guidelines discuss a maximum distance of 40". That equates to a screen nearly 6 feet high, which would be a 12 foot 16:9 monitor.

If your problem is that your monitor is larger than 12 feet diagonally, you can probably afford regular chiropractic care and good glasses, and you will have the satisfaction of knowing that your monitor makes other people drool.