As Darrel wrote earlier, anything above the highest value (1000 in this case) will cause 100% dutycycle - all that's happening is that you get a flat spot at the very top of the otherwise linear curve. Only you know if that will be an actual problem for you.
If you want to scale your 0-1023 ADC input value to 0-1000 dutycycle value you could do something like DutyCycle=ADCValue**64063 or, for 8bit resolution, DutyCycle=ADCValue**64251.

/Henrik.