The theory behind "expose to the right" is sound. However I have a problem with the language used to describe it. Exposing to the right isn't overexposing it's getting the exposure right.
Back in the day (film) folks would talk about different films having exposure latitude. What that really meant was if you got the exposure wrong, but close enough, you could still save your *ss and get a print you could sell to someone who didn't know any better. When I used to hear my students in class say film had latitude, I would respond with, "Film has latitude for people who think excellence has latitude."
A correct exposure places the diffuse highlight just below white. PERIOD. If you do that you've got a correct exposure. The problem today with digital is it's really hard to know if you've done that because you can't see it. Again back in the day, when you got the lighting right and nailed the exposure and didn't screw up the chemicals you could look at that finished negative or transparency and know you'd nailed it. It took some time to learn to read a neg but after a few thousand you got to where you could see it. You knew when it was right.
Today you have to look at an interpretation of what your camera captured. Even a RAW file in LR without any applied adjustments is being interpreted by the software. LR and/or ACR is applying tonal adjustment curves to your data just to get it on the screen for you. You look at that image and say, Oops! looks like I overexposed. Did you? How do you know you overexposed? You're looking at a software interpreted image. Your original data has already been manipulated. Maybe you did just fine and the software is screwing up. All of the software is programmed to manipulate your data in accordance with assumptions that in many cases may not fit your intention. Just to see your photo in LR your data has to undergo manipulation based on the programers/engineers adopted assumptions.
Going back to JW's original question -- How to interpret LR's histogram -- begs another question: How has LR interpreted the RAW data?
I switch between different RAW converters with some regularity. In principle, you'd think that a RAW converter would display your photo without first adjusting it -- after all you shot the photo in RAW so YOU could adjust it. Look at this example:
I opened the same file "as shot" in three different RAW converters: Adobe Camera RAW, Capture One and Raw Therapee. I pasted in each programs histogram window for reference. C1 right out of the gate says I overexposed this shot. The highlight clipping warning is active in the flowers. ACR says I got a pretty good exposure and RT says I underexposed the photo quite a lot. How about that: I underexposed, overexposed and correctly exposed all with the same RAW file!
Above I said this, "Just to see your photo in LR your data has to undergo manipulation based on the programers/engineers adopted assumptions." Well photography has standards. It's fair to expect that those software engineers are adhering to industry standards isn't it? Look at the above photos.
This is not a encouraging answer. Never again can we put a loupe over that neg or transparency on the light table and say, "nailed it!" So first step in interpreting LR's histogram is to recognize that LR's histogram is itself an interpretation that you should immediately hold suspicious.
Joe