My project involves developing a multi-mission modular satellite bus. I have couple of payloads under consideration and have the necessary sensor details. Based on that I am trying to figure the pointing and stability requirements of the satellite.
The pointing stability requirement of a platform is dictated by the payload that will use the platform.
What will the payload do? What pointing accuracy and stability does it require to function well and do what it is supposed to do?
Just as one particular example, if it has a camera and will photograph the Earth, then you need to look into the magnification - how many pixels per degree, and the exposure time you'll need. Do you need to spin the satellite in order to slew the field of view in order to track the image of the Earth's surface with its apparent motion of 7 kilometers per second, or do you just need to hold steady?
Start with the purpose of the mission and work backwards to find technical requirements.
If you want to include some more information about the payload's mission and function, that might help attract a more helpful answer.
However: you might have meant to ask about the pointing and stability performance rather than requirement. In that case you should revise the question.
In any event, please leave a comment here if/when you revise the question.