How do I find the firing angle of SCR practically?
It would help if you had some dimensional analysis in there. 

Presumably (though you didn't bother filling in your profile so it's hard to tell) you're in a 50Hz country, so each half-cycle is 180 degrees and 10 milliseconds. Hmmm.. 180°, 10ms ... anything we could do with those? Maybe to find °/ms?