By measuring to the back of point/end of arrow shaft, it's consistent across all arrows. One would lose that consistency if one measured to the end of the points, which, as you note, can vary in length. However, that minor difference in length wouldn''t amount to more than 1-2% of FOC.
For your own purposes, you can do it any way you like, as long as you do it consistently. Of course, your results won't be comparable to others unless you measure to back of point.
Just reread the thread. McDave already said all this.