I've been trying to make a spell whose missile always travels the same distance from the caster (in the direction it was cast), no matter where it was targeted. I've been messing with the launch and impact site operations, and nothing seems to work.
The projectile will have some sort of Mover associated with it. Under Motion Phases+, there is a field at the end called Timeout; if the projectile fails to find a target it will be destroyed after these X seconds. From this you can derive a desired distance.
Use a create persistent effect to set the target point for the launch effect. Set the Location to Source Unit, the Periodic Effect to your launch effect, and the Periodic Offset to (0, -y) where y is the distance you want the projectile to travel.
You probably either didn't set the Location correctly or you used anything but periodic offset. Initial/expire offset don't work correctly, I think they don't take facing into account while periodic offset does.