Desing Problem with Single Axis Solar Tracking System
Here is my interpretation of what you need.



U1-U4 compare the LDR values to detect when the left or middle sensors are the maximums. Depending on the outputs of those the voltage divider with three pots is adjusted to generate a demand POSITION voltage. This demand is capacitivly filtered to produce a slower swing between demand locations and make the servo move more smoothly. 

The 555 circuit generates a 20mS sawtooth waveform. This is compared with the POSITION voltage to generate the appropriate width servo pulses by U5.

The reset circuit beside the 555 is supposed to ensure all the capacitors have had time to settle before the servo pulses start being generated.

CALIBRATION


Monitor servo pulses with a scope and adjust the saw-tooth current with pot X4 to set the pulse period to approximately 20mS. 
Illuminate the left sensor LDRL, or short it out, and adjust pot X7 to position the servo to the correct left position.
Illuminate the middle sensor LDRM, or short it out, and adjust pot X6 to position the servo to the correct centre position.
Illuminate the right sensor LDRR, or short it out, and adjust pot X5 to position the servo to the correct right position.


ADDITION

If you have to get rid of the 555, this may be your best bet.