When scolding a misbehaving child, we tell them to be iikagen:
いい加減にしなさい。
But we can also tell a misbehaving child to stop being iikagen, or describe something bad as iikagen:
もういい加減でやめなさい。
彼のロシア語はいい加減なものだ。
I understand that いい加減に and いい加減で/な have different meanings, but how did those evolve from a word that literally/originally means "add and substract"?
Let me show one interesting concept mostly used by scientists, Goldilocks principle.
Quote from wikipedia
The Goldilocks principle is named by analogy to the children's story "The Three Bears", in which a little girl named Goldilocks tastes three different bowls of porridge and finds that she prefers porridge that is neither too hot nor too cold, but has just the right temperature. The concept of "just the right amount" is easily understood and applied to a wide range of disciplines, including developmental psychology, biology, astronomy, economics and engineering.
This exactly shows the original meaning of いい加減. いい加減 is, in other words, kind of tuning. "Tuning" might be the virtue of the traditional Japanese. In fact, many traditional Japanese culture have capability of "tuning" or "adjusting", such as chopsticks (compared to knives and forks), kimono (compared to western clothes), fusuma (compared to doors), etc...
There is also a famous maxim in Japan, 過ぎたるは猶及ばざるが如し, which means "too much" is almost equal to "too less". This also shows the virtue of tuning and it indicates that adding and subtracting is important.
Regarding the meaning shifting, the link in @BJCUAI comment is great!!
いい加減 means "proper degree". It doesn't mean "add and subtract".
This words came from "good degree of add and subtract".
In "いい加減にしなさい。", "もういい加減でやめなさい。" , "いい加減" is used as "proper degree".
In "彼のロシア語はいい加減なものだ。", "いい加減" means "coarse" that contains negative context.
