Kha'zix's ultimate could use a QoL tweak

There are many instances where I only need to use my ult once (or twice with evolution) and feel like I end up wasting an opportunity because it triggers an 80 second cooldown on an ability whose cooldown is balanced around you using all of its "casts" and possibly not being able to use it in the next fight you have due to that.

What if instead Kha'zix's ultimate worked on a charge system (like akali's/vel'koz/etc). *Current Ult

1. (Unevolved) Cast void assault and immediately go into stealth, able to cast a second time within 15 seconds of the first cast (80 second cooldown)
2. (Evolved) Cast void assault and immediately go into stealth, able to cast two times within 15 seconds of the first cast (80 second cooldown)

*Proposed change:

1. (Unevolved) generates a charge of void assault every 40 seconds (80/2) and caps at 2 (still an 80 second cooldown if you use both, but doesn't punish you for not using both)
2. (Evolved) generates a charge of void assault every 26.66 seconds (80/3) and caps at 3 (still an 80 second cooldown if all 3 are used, but actually rewards you for putting your point into evolving it (faster stealth charges) as well as not letting the extra charges go to waste)

What do you guys think?