Yea I found that in my Clymers manual to, but it shows how to measure but I have not found what the distance value should be.
One additional note on this. The measurement distance is only good if using factory springs. In my case I used Racetech springs and their volume is different that stock. The volume of the Racetech is less than stock. Meaning that if I was to use the stock measurement I would have to much air space. Hence the reason for using a volume measure rather than a distance measure.
The numbers are in the Honda manual, and they're different for L or R, and ABS1, ABS2, non-ABS. Don't have it with me, I'm at work, but I think you don't really need the numbers anyway.
You're right about the displacement volume of springs being different, but the oil volume specification Honda provides is based upon using the stock springs, same as the height specification. Its just a different way to measure to get to the same result. I think the reason they specify both is if you don't have a graduated cylinder to measure the oil volume, you can use the fork tube as a graduated cylinder and they give you the measurement to know how much 'volume' of oil to put in. Some people prefer to use the height method because if you don't get every last cc of old oil out, then adding the specified volume of oil back into the tube could result in slightly overfilling it.
So if you change to a different spring the volume method isn't any better than the height method in that regard. The height will be different if you put in the Honda specified volume of oil, and insert a different spring.
But once you change the springs I doubt the air volume needs to be exactly the same as stock anyway, so trying to precisely replicate that seems unnecessary. And, different riders need different fork setups, so there's really no right/wrong setting to begin with. As long as you measure either height or volume with your given springs, you'll be able to replicate the desired setting next time around when you change the fork oil.