"Is there a proper height that dock lines should be secured"
Not particularly. It is desirable to secure the fixed part such that at tidal mid-point the line is parallel to the water, but this is only so that you can secure the boat as snugly as possible throughout the tidal range. This is not always possible or practical. A line that has it's fixed part significantly higher or lower than this point will be excessively tight at one end to the tide range, and excessively loose at the other. Looking at your home port, I'd guess your daily tidal range to be less than 4 ft. If this is true, then "somewhere sort of close" should be plenty good enough. Ther range is not great enough to really make it matter.
In my opinion, of course.