There is no general rule of thumb for penetration in a class, but there is a very blurry line for what is acceptable and what is not for certain tiers.
This is just gonna be a list of what pen is going to be needed to go through heavies frontally at each tier:
At Tier 10, you can pen nearly everything frontally with 330mm of HEAT or 320mm of APCR. If that means running Calibrated Shells to get that pen, that's usually worth it for most players. As for standard shells, 260mm of AP pen is pretty good for taking care of lower plates and any other weak spots frontally.
At Tier 9, its helpful to have 330/320 pen to pen Tier 10's, but that's going to be overkill for Tier 8's. You can make do with 300mm HEAT/290 APCR pen. AP should be around 240mm.
At Tier 8, I would recommend having at least 250mm of HEAT or 240mm of APCR. Tier 8 heavies are surprisingly strong now. 220mm of AP pen is plenty, anything above 200mm will work.
I don't play enough Tier 5-7 to really know what pen is required, but I'll give it my best shot. Basically the lower tier's prammo should be roughly equal to the higher tier's standard.
Tier 7: Basically take the AP from Tier 8 and apply that to Tier 7 prammo. Most tanks at this tier don't have HEAT, so 200mm of APCR will do and 220mm of APCR is good. 170mm of AP pen is fine.
Tier 6: 170mm of APCR, 130mm of AP.
Tier 5: 130mm of APCR, 100mm of AP.
To be honest, I don't really look at penetration all that much. Credits have never been and never will be a reason for me to not spam prammo when needed. My mentality is that I do whatever I need to do to give me the best chances at winning. 99% of the time, I start by firing standard shells and I judge each shot case-by-case to see what shell makes the most sense. I usually will make a reasonable effort to pen with standard shells because more damage, but if I have to take unreasonable shots to pen with standard, it's time to load the gold. If I still can't pen, I either switch to HE (rarely) or find a new target/relocate. Occasionally, I'll still relocate or pick a different target even if I can pen a tank with gold that's in front of me, because sometimes its more important to take out a certain tank that is more dangerous to my team rather than just taking every damage opportunity that I have.
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