In 8-man HM, you will usually get three Kephess, although this depends largely on how good your DPS are and whether your melees switch to Kephess or stay on the boss.
Like the other posters already said, the Kephess are not percentage based but are constantly spawned by the boss on a 30 second cooldown.
It sounds like all your DPS are staying on the boss and not focussing on the Kephess clones. With your gear, I can hardly believe you can get the boss to 50% when the second Kephess spawns; the boss should be around 75% then. Are you sure you are in 8-man HM?
From your description, it seems like your DPS are a little too afraid of the lightning orbs and they want to burn the boss ASAP to get as little orbs as possible. I suggest that every DPS learns how to pick up the orbs better (move away from the group as soon as you get the white orb connection, tell via voice-over you have an orb so that healers can top that player off, stop DPSing the boss/Kephess, but target the orb and walk into it at ~18 stacks). The DPS need to be able to pick up orbs continuously, even the same DPS getting two orbs in a row.
Four orbs are way too little; if you're having trouble with only four orbs, I think the damage dealers are the reason you are failing. When your DPS learn how to handle the orbs, it doesn't matter whether you get four or ten orbs; you won't event notice them anymore.
As always, it would help seeing a video of your group to determine exactly what you're doing wrong. Since you are asking specifically on how to handle the Kephess phase, here is how my group is doing it:
In our group, one tank always takes the Kephess clones. He will also be the first tank that takes Brontes.
The second tank only takes Brontes when the first tank is on the Kephess.
We always have three DPS on Kephess and on DPS (in our case the melee) stay on Brontes.
This is the exact order in how we handle the phase:
- As soon as boss appears, first tank taunts the boss
- Once the Arcing Assault channel is done and the first tank has four stacks, the second tank taunts
- A few seconds later, the first Kephess will spawn. First tank jumps to Kephess and points him to the side
- Once the tank got one or two hits on Kephess, he tells DPS to switch to Kephess. We have three rDPS on Kephess and on mDPS staying on the boss
- As soon as Kephess is down, first tank leaps back to Brontes and taunts so that the second tank can lose his stacks. Second tank stands behind Kephess then.
- Once the boss finishes channeling the Arcing Assault, we once again do a tank swap. Second tank stays on the boss, first tank leaps to Kephess
- Ranged DPS once again switch to Kephess. Once Kephess is down, Brontes should be around 65% HP and the energy spheres start appearing. First tank once again takes Brontes so that second tank can lose stacks.
- As soon as Arcing Assault finishes, second tank takes the boss and first tank leaps to the third Kephess.
- This time, all DPS, including the melee, switch to Kephess. We do a damage stop on the boss (he is usually around 52% HP) to make sure Kephess is dead before the droid phase starts.
- As soon as Kephess is down, we push the boss to 50% and the next phase starts.
- If another energy sphere appears right before Brontes gets to 50%, you can have your tanks get that orb (like written above - Guardian does taunt, saber reflect and leap) so that all four DPS can focus on the droids.