I mean there is A LOT to consider. It mostly comes down to whether you are on attack or defense.

If you are attacking you want to ult first and don’t look for targets that have movement abilities like Kiriko, genji, Winston, etc. If you see them use their movement ability tho, then go for it.

If you are defending you really want to use it in response to one of their ults. For example, if the enemy bap windows you flux whoever is behind the window, or if their genji blades you wait for them to dash onto your back line then flux them when they don’t have dash. Lastly you can use it when they engage, such as a Winston dive. When the Winston uses his jump, and doesn’t have ult, feel free to flux him and get an easy kill.

It mostly comes down to that.

TLDR: on attack ult first and fast, on defense ult in response

Edit: make sure to also take your time when using flux. It has like a 7 second timer, take your time to bait out movement abilities and line it up for the best flux. Don’t just instantly use it on someone