Enchanted Crystal Arrow:
Active: Ashe fires a giant arrow in a straight line. If it hits an enemy champion, it will deal magic damage and stun that champion for 1-3.5 second(s), based on the distance the arrow traveled.
As the description from Ashe's ultimate says
[...] based on the distance the arrow traveled.. This means, that the stun duration is calculated from the position where you used the ability, because after firing the arrow, your movement won't affect the distance the arrow travels.
This also adds up with the new description of Nida's Q:
Active: Nidalee hurls her javelin forward in a line, dealing magic damage to the first enemy it hits. After having traveled 525 units, the javelin's damage will start rapidly increasing. The damage modifier is capped at 250% at maximum range.
Here it also states the damage multiplier is calculated from the spear travelling a certain distance. This adds up, with the fact that the actualy position of your champion when your spear hits does not matter.