This guide was made pre-2.0, and hasn't been updated much for the full lethality part as it is not a viable spec for pve anymore,
Well, it says "major update since 2.0" so I kind of assumed it was going to be udpated.
This guide was written for the Lethality/Engineering hybrid, which doesn't have the takedown procs. If you're referring to 30%< hp takedown I'd say use it as a filler if you've got a lot of energy (like cull - sos takedown - cull). Might not be worth sacrificing EP or Lazed snipe for it though. Never seen any real numbers on what is most effective but I'd assume so.
The proc makes so it resets Takedown cd and you are able to use on targets regardless of their health. As for the energy issue, that's why I'm keeping the "old" armorings for bonus set since they provide a significant reduction of Takedown's energy cost.
Currently, tooltip on my sniper shows:
Explosive probe: Energy 25, damage 4288-4395, 30 seconds cooldown
Lazed snipe: Energy 20, damage 3451-3639 (crit multiplier 67,40), 1 minute cooldown
Takedown: Energy 8, damage 4511-4757. Cooldown, well, pretty much everytime after a Cull. With both dots up, I don't remember ever not getting a proc for it.
If energy is an issue and I have to choose, I would skip EP and use Takedown if possible tbh