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3 of 3 added suggestion that the word might come from genealogy

Single word for the height above something

When we talk about height, usually we mean the distance from an object to an external zero reference such as the ground. I'm looking for a single word that describes how much height is resting on or hanging from an object, where the object itself is the zero reference. For example:

If the top floor has a ____ of zero, then the third floor of a five-story building has a ____ of two.

If there's a word for this concept in mathematics, that would be ideal. Or a word from genealogy expressing the number of generations of descendants a person has. A word from architecture or shipbuilding might also fit. I considered the word freeboard, which is roughly the height of a ship above the waterline, but the reference always seems to be the waterline. I don't think you'd speak of the freeboard above a particular deck. I also thought about the word load, but I don't want to express the total mass or volume of stuff, only the dimension of height.