Groove:
Did the OP ask about analogWrite?
No.
Did the OP ask about PWM pins?
No.The OP asked how to calculate the correct PWM to steer a servo.
And the answer is that you don't need to calculate the PWM, you calculate the required position and let the Servo library deal with generating the PWM. If you're trying to use an Arduino to control a servo and generating PWM output yourself, you're making the job much harder than necessary.
Getting back to the OP's question, I can see two approaches to solve the problem.
Knowing the aircraft position and orientation, it is possible to calculate the relative direction to a specified point on the ground using trig. The calculations are non-trivial.
There is an open source Arducopter project which provides many features including the ability to control a camera gimbal to point the camera at a fixed point. Even though the platform in this case is not a quadrotor, it may be practical to take the tried and tested sensing and control hardware and software and use that as the basis for a camera steering system. (I suspect it would also be a damned good start towards an autonomous autopilot, if there was any interest in that sort of thing.)