The exact meaning is slightly ambiguous. When I hear "I'm calling it", I don't personally think of "I'm calling it a day". The character is not exactly saying, "Well, I'm stopping work for the day"; the overall meaning in this case seems to be something like "I am declaring my decision."
One meaning of to call something is to announce, proclaim, or declare it, and this is used in a number of phrases like
- "calling it a day" meaning proclaiming that work is done for now
- "calling your shots" meaning announcing in advance what play you will make in the game of pool (or sometimes in other sports or games)
- "calling a code" meaning declaring a medical condition
The last one is actually what I am most reminded of. When a patient dies despite attempts to save their life, it is standard (at least on TV and in movies!) for the doctor to say, "I'm calling it." (where it is a code in the medical sense) meaning, roughly, "I am declaring that nothing more can be done for this person. Nurse, please note the time of death so we can put it on the death certificate."
So when the character says "I'm calling it", what I interpret it to mean is "I am announcing my final decision on this matter."