Questions for line following robot

I was doing the project of line following robot and I met a question about PID controller.
The algorithm I used is speed_right=central_speed+PID and speed_left=central_speed-PID
The central speed is defined by user and PID is the feedback of sensor array.

So, lets say the central speed is 30 and PID gives 10 so the final speed becomes 20 and 40 thus the direction is significantly changed. But if I set the speed as 1000 then it becomes 990 and 1010 so the direction is barely changed!

I want to ask if there is any math model for it or relationship between central speed and PID value that I need to know?

The output from the PID model is proportional to the error - the difference between the actual value and the desired value, whatever those values are.

Without knowing what you Input, Setpoint, and Output values are, there is no generic answer to your question.

You could map the output value to another range, if you want. If you think the output means %, and you want it to be actual speed differential, then scale accordingly.