flywheel timing mark

My Dodge 360 engine's timing can only be checked looking downward at
the flywheel from an inspection hole inside the van. The smog specs
call for 6 degrees retarded. Since there are no reference marks at the
inspection hole except for a "centering" line on the hole, can someone
tell me how many fractions of an inch I should eyeball the flywheel
indicator mark from "center?" I assume the smog check station also
have to approximate the mark. Last time I had the beast smogged they
had to check the timing by this method.

Any input would be appreciated.

Dick