hi,
For a soft start i suppose you increase Vin from 0v to 5v-->0 to 1023 (DAC conversion)
For 0v you need the max delay pulse :60-70 (need to be adjust because that cause flicking if you are over 10ms period)
for 5v you need the minimum delay for 220v :2
the final code is:
OCR1A = map(instelwaarde, 0, 1023, 70, 2);
regards