It's easy to divide a sine wave (curve) into any number of slices so that area of all slices become equal. It's a problem of 'definite integral'. Area of a sine curve with amplitude 'a'(peak) between '0' and 'pi' is '2a'.
Now 2a/40 is the area what you want !!
I think this much hint is ample for the problem to be solved.
You can take help of any Calculas book where "area under curve" is described.
Don't hesitate to ask if you still cannot solve the problem .

AGAIN please let me know about the circuit you used for detecting the zero cross of the mains voltage

 

I have moved your post to its own thread., as the old one seems to have been abandoned - http://forum.allaboutcircuits.com/showthread.php?t=25794&page=3

Google finds many zero crossing detectors. this is one - http://sound.westhost.com/appnotes/an005.htm

It is very important to use a transformer to provide a safe isolation from the line voltage, and to drop the voltage to a reasonable level.

 

Parhaps the "SUPER MODERATOR" failed to go through and realise my answer.
Now its a be-headed thread.
SORRY!!

If the thread : http://forum.allaboutcircuits.com/showthread.php?t=25794 was abandoned, why it's left opened ?

 

That is beside the point. If the circuit linked to does not satisfy your needs, try Googling "zero crossing circuit".