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As noticed in https://github.com/w3c/webvtt/pull/100#issuecomment-45607413 It says "minus half of the cue box height" before "Apply the terms of the CSS specifications", which means that the height is not known yet. That would have to go under "Adjust the positions of boxes according to the appropriate steps from the following list" instead.
Fixed in https://github.com/w3c/webvtt/pull/116