"Eide" wrote: (clip) Site through the strings onto the
hemisphere and make a pencil mark every 6 to 8 inches.(clip)
^^^^^^^^^^^^^
I have a level with a laser beam which shoots out one end. If this were
mounted on a tripod, and adjusted to level, and axis vertical, then
rotating it would sweep the spot across the curved surface you intend to
cut. As suggested above, make pencil marks as close together as you desire.

Or, you could mount a rifle-scope-sight on a tripod, and do the same thing
by directing someone with a pencil to make the marks.

Or you could use a surveyor's transit or level to do the same thing

I have, but have never used, a pair of transparent tubes that can be used
with a water-filled garden hose, that can be used to carry a level
reference. Probably less accurate than the other methods, though.