Control servo motor via PID to stabilize a robot wheel using IMU
To do this "scientifically", you need to decide exactly what physical quantity the output of your PID controller represents, and in exactly what units. Does it represent course error, heading error, steering angle or something else?

Also, what physical quantity does the input signal to the servo represent? Steering angle?

If both quantities represent, say, steering angle, then you're mostly done. You just need to apply a scale factor to convert the units.

But if they represent different physical quantities to begin with, you need to figure out a way to translate from one to the other. This is called "kinematics" (or sometimes "reverse kinematics", depending on how your control algorithm works). It's basically a mathematical description of how the various physical quantities (mainly angles and distances, but could also include forces, masses and velocities) are related in your system.