Thanks! Although I looked into that thread beforehand and found only 'penetration chance are capped at 1' but the exact relation of armor and penetration remained unclear.
I often use this file to find the values:
https://drive.google.com/file/d/0B7gwy65JLbSRMEJ3M2ZPandMMW8/view
(download it on your computer, then open it, otherwise it won't show up correctly)
Note: I am uncertain if all the values in the file still applies, due to recent balance changes, but for a quick overview without needing the mod tools, this is a fantastic source.
Generally, the rule of thumb is:
Chance to penetrate in percentage = (penetration values of weapon / armor values of target) *100.0f
e.g. (some weapon with 120 penetration / some tank with 290 armor)*100 = 41% chance to penetrate and inflict damage.
The values are capped at 1, this means, in layman's terms, that the values are capped at 100% chance to penetrate. Some users prefer to use percentage to talk about penetration values, some others don't. Large parts of the game-engine works with float values between 0-1 (so 41% penetration is equivalent to 0.41). It's all about rolling dices at this point; even with only 41% chance to penetrate, you may get lucky and have all shots in a row penetrating and damaging a target - or the complete opposite if you're unlucky.
Keep in mind that tanks are merely divided into front/back armor and that side hits may either count as front hit or back hit, depending on where exactly the shell hits.