According to the mechanics area on playdota.com when having negative armor the following formula is used to calculate the increased damage:

with a cap for negative armor of -20. This results in a maximum of 71% increased damage through negative armor.

DotA2 whatsoever seems to use a different coefficient than 0.94 also it seems it is not capped to -20. I did some quick tests with Ogre Magi on level 25 with 6 hots (12 armor) and several items and spells to reduce its armor (melt, dazzle ult, venge wave, slaradar ult, desolator, medalion and AC) resulting in negative armor values of around -44. So in total 12 - 44 = -32. This should result in -71%! The tooltip showed 65% extra damage, so there must be something wrong (or of course playdota.com is wrong on that!)

Code:

1 - 0.94 ^ (-armor)

DotA2 whatsoever seems to use a different coefficient than 0.94 also it seems it is not capped to -20. I did some quick tests with Ogre Magi on level 25 with 6 hots (12 armor) and several items and spells to reduce its armor (melt, dazzle ult, venge wave, slaradar ult, desolator, medalion and AC) resulting in negative armor values of around -44. So in total 12 - 44 = -32. This should result in -71%! The tooltip showed 65% extra damage, so there must be something wrong (or of course playdota.com is wrong on that!)

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