Originally Posted by NOS88

If you mean standard in terms of a professional, widely accepted organization that sets standards such as National Institute of Standards and Technology, setting a standard for this, I haven't a clue.

However, given seatposts are not all the same size (think aero posts), I can't see another way to do it with any consistency. Additionally, when I've completed forms for custom built bikes, or forms to record your current setup, it's been measured this way.