The Chieftain was a medium tank, much to my surprise.
This a Chieftain tank. The books say the front hull is 120mm thick. The bore of the gun is 120mm, the hull armor is obviously quite a bit less, about 85mm at 72° (275mm horizontally). The Chieftain was therefore only pretending to be a heavy tank back in the Cold War. Wot a beastly rotten swiz!
So this Russian diagram is correct:
The Stillbrew upgrade (1986) is a 150mm steel plate backed by 50mm of rubber:
Pictures from tank-net.com.
T-72 seems to be thicker on the turret than Chieftain.