So evolving Kha'Zix in this order is usually how I go about his Ult.

i always evolve ult, then leap, then Q or W. I thought that's how everyone did it

The only thing I'd suggest otherwise is that if you're getting stuck into teamfights W evolve is better than R for the poke damage.

i feel like Q then E are maditory because the dammage from Q is core and the E while core too is not very useful if you cant kill them W and R seem interchageable depending if you are able to get 1v1 or are teamfighting

Depending on how the game is going you can get a lot done with evolving E first, because it enables some pretty strong tower dives