there are timing lights that you can set advance and retard on. what you do is say you want to set the timing at 20 degrees advanced for some reason.. set the timing light at 20 degrees advanced, then it will line up the TDC mark with the timing mark on the timing cover when it is in time.

If you have a normal inductive timing light, just use the three marks as reference

 

i like to use those three marks because one time i tried to do it the other way and it ended up all retarded. i told my cuzzin and he's all you probably did it wrong. he went to check it and it came out to be freakin 14 degrees. and i was supposed to be at 18