Online Poker rooms use a formula known as Monthly Gross Revenue (MGR) or net rake. Your rakeback is calculated using this figure.
For example with a rakeback percentage of 30% if your MGR equals $1000 you will receive $300 rake back.

When calculating your MGR some rakeback poker rooms may subtract any bonuses earned during the month. Most poker rooms will include tournament fees but not all do so please check.

Your MGR is also determined by the method of  rake a poker room uses. Some poker rooms use what is called contributed rake where you must participate in the pot to have rake credited. Others use the dealt method where you are credited with rake in every hand where you’re dealt cards.