I'm not sure I follow what you mean by 'armoured formation'.
Like a Tank formation?
If we are to take Warhammer 40,000 as an example, in the standard Force Organisation chart (which was designed to allow the fielding of one Battle Company of Marines, plus some extra support choices), for each full roster you took, you could take 3 battle tanks like Predators, Vindicators or Land Raiders.
Which would sorta imply that for each Battle Company in your army, you should take one tank formation, be it Predator, Vindicator or Land Raider to remain 'fluffy'.
Kinda flimsy really.

Overwhelmingly in the background Marines will field whatever tool they feel is best for the job at hand.
Imperial Armour III has a full Epic order of battle for one of its scenarios, in which the Marines take:
- Tactical Formation
- Captain
- Dreadnought
- Tactical Formation
- Chaplain
- Dreadnought
- Devestator Formation
- 4x Razorbacks
- Assault Formation
Whirlwind Formation
- 3x Whirlwinds
- 1x Hyperios (what we call a 'Hunter')
Land Raider Detachment
- 3x Land Raiders
Thunderhawk Gunship
Thunderhawk Gunship
Battle Barge
----
Which is roughly 2700pts of 'fluffy' 2nd (battle) Company Marines.
Roughly.

To take a 6th (Tactical) Company army, it recommends:
- Drop the Devestator, Assault and Land Raider Detachments and replace them with 3x Tactical formations.
- Add 2x formations of 4 Land Speeder Tornados.