You can easily scale in Arduino code with the map() function.
const int MIN_LIGHT 100;
const int MAX_LIGHT 700;
int light_reading = analogRead(A0);
// Convert MIN reading (100) -> MAX reading (700) to a range 0->100.
int percentage_bright = map(light_reading, MIN_LIGHT, MAX_LIGHT, 0, 100);
But you may need some calibration of your system to handle different ambient light situations. What reading do you get in direct sunlight for example?
So then I assume you're using PWM (Pulse Width Modulation) to drive the LED. This requires a value between 0 & 255. Once again we can use the map() function.
// Convert light-value % (0-100) to PWM speed (0-255)
int led_brightness = map(percentage_bright, 0, 100, 0, 255);
Obviously these bits of code can be reduced down to a single line - mapping directly from the reading to the desired result.