It is.......
calculated through simulations.
It is like Basic Strategy, you have to trust it!
Average adventage that you can gain counting cards is about 1% (by the way 2% is great great great adventage, that happens very rarely)
Adentage that you gain counting cards is calculated by putting together
Number of decks
Rules of game
Deck penetration
Bet Spread
And type of counting system you use
There are great programs that will calculate that for you.