What is the maximum distance for Janus's ultimate when attempting to unlock the No Scope award?

In patch 3.9 Janus can now achieve the No Scope award:

As Janus hit at least two enemy gods with your ultimate at max distance.

Janus's ult appears to travel through the entire map. It casts portals for the first 500 units, but appears to do damage throughout its path. I note that:

Distance Scaling: 200% of your Magical Power at distance greater than 800.

...which suggests that it goes beyond 800 units (is that correct?).

I have been playing Arena, firing the ult from my fountain into the opposite minion portal (i.e. where my team's minions exit the map). I have hit multiple enemies in this area. This has not given me the award.

According to this question Arena is around 500 units long. However, someone has unlocked it on Clash at what appears to be a smaller distance than my Arena efforts.

What actually counts as "max distance" for this award?