If we talk about a fleet engagement, cruiser clash, exterminatus, planetary assault:
Ofcourse a pair of Devastations. That's a given. With the PK that's already 10 lances at 60cm.
Two small escort squadrons at the back of the fleet. They don't do anything except coverings the PK's back.
You can go in pretty close formation, keep an open straight fire path (armageddon gun fires straigh iirc not in arc). In an exterminatus the enemy will focus on taking out the Planet Killer. That's hard. So make him worried with the rest of your fleet.