I do like the zone Idea myself, though I'd suggest the health damage percentages to be 90%, 60% and 30% respectively, then it's nice and easy being 3 PvP capped hits, 2 PvP capped hits and 1 PvP capped hit.
I'd probably say Zone 1 wants to be a relatively small radius, couple of meters at most, with Zone 2 stretching to a few meters outside that and Zone 3 about the same as the current radius of Orbital strikes. Meaning the majority of people would be in Zone 3, while a few would get caught in Zone 2, and very few if any would get caught in Zone 1.
Not sure I like the idea of a Nano drain on it though.
Huge Knockback on it could be real fun, flying miles through the air and landing half a zone away would be an interesting way of managing it.
As for konckback in relation to the zones, I'd recommend the following:
Zone 1: No knockback, the blast is right on top driving them to the ground, not away from the blast.
Zone 2: Huge knockback, the blast is right next to the player, the force being thrown at them enough to send them a great distance away from the blast. Short stun on landing (1-2 seconds) followed by a moderate snare (dizzyness), should take a few minutes for the players to return to the field.
Zone 3: Smaller Knockback, the blast isn't that close the the player, but still close enough to send the player flying a short way off, small snare after the knockback, these people should be able to rejoin the fight relatively quickly.
Pretty Picture to help Illustrate things:
Pretty Picture KeyCode:/ \ ! / \ / . . \ ! / . . \ / . . \ ! / . . \ / . .\!/. . \
Zone 1 and Blast: !
Zone 2: \ /
Zone 3: ...