Is it the original motor?
It depends on what the mill is, compression, cam, etc... could be anywhere from 4 btdc to 12 btdc....
Most sb chevy's like at least 32 total degrees of timing... That's what the timing is at when the centrifical weights are at full at advance.. Usually they're all in at about 3000 rpm... modded motors with bigger cams, etc, can take MUCH more total timing...
Does your light have an advance knob? That makes it simple to check total timing.... IMO, total timing is the critical factor, how it wants to run in it's operating rpm range... much rather it run balls on, at higher rpm than at idle...
Another method many employ for timing is advance it in small increments, test run it, and just when it's starts to ping, back it off 2 degrees...