When the instructions on a microwave dinner box advise you to "pull back corner of film to vent," you typically need to pull back a small portion of the film. The purpose of this step is to create a small opening to allow steam to escape during the cooking process, preventing excessive pressure buildup.
The exact amount you need to pull back may vary depending on the specific packaging and brand. However, a general guideline is to pull back about 1 inch (2.5 centimeters) of the film from one corner. This should be sufficient to create a venting opening without fully removing the film.
Make sure to follow the instructions on the packaging carefully to ensure proper venting and even cooking of the microwave dinner.