At Omega Too, we hear " How high should a light hang over a dining table?" quite a bit. Here's what we say:

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We normally recommend hanging a pendant lamp 28 to 32 inches from the top of the table surface to the bottom of the light. { Some folks like to say 60" from the floor, but not all eating surfaces are 32" high.} The width and length of your table will help determine how high to hang it as well: a larger table can support a pendant being a bit higher, whereas a smaller table looks best with a lower-hung, and skinner, pendant. Contemporary spaces look great with a lower-hung pendant light, but if it is a "drum" style, keep in mind that hanging it on the higher side you should consider adding a diffuser. Nobody likes to see the light bulbs. Or the spots you se when you blink after looking at them.
Also, keep in mind your user. If you live with a family of basketball players, or dad is the one who clears the table every night, and he has a habit of bonking his head a lot, perhaps you want to toss the rules out the window and go for custom ergonomics.