You have the option to enter the commission you pay at the different exchanges on the settings page, in percent. The commission you enter will automatically be deducted from from the bets you win using that particular bookie and will therefore affect both your result and stats to show more realistic numbers.
If the commission you pay changes for a bookie, just change it on the settings page and all future bets you place at that bookie will use the new percentage in the calculations. Older bets already settled will not be affected.
Regular bookies normally don’t charge commission, and in that case commission is set to 0, which is also the default value.