dear azhar..try this and it works.... i mean it, i have done it myself...
use a comparator as a zero crossing detector.. which detects when the mains supply crosses zero. when the comparator toggles, give the output as a trigger to a monostable circuit. as for the monostable... calculate the delay required by you..(5% duty cycle i presume) and design the monostable accordingly. output of the monostable should be given as trigger to your SCR.
you may as..why the zero detector? its simply for synchronization with the mains cycle. and yes... dont forget to include an isolation circuit in between..
try this and let me know what happened.
best of luck
tushar