Fan,
You MUST have a 50Hz PWM drive. It MUST vary the duty cycle from about 1 to 2 mSec. It WILL not work otherwise. It only works now from some nasty non-linearity in your servo I suspect !

First get nice code that will take a digit and turn that into a position. Enter numbers from the keyboard: confirm that the servo moves repeatably to where you want it. Then build code that sweeps the digit slowly to your desired value (until your interrupt says STOP !)

Build up in stages: TEST: TEST : TEST

Steve