If the voltage drop is 1.8 for the return path, then the resistance is 0.015 ohm (1.8 V / 120 V). For a nominal 120 W bulb the current flow is ~1 A (~120 VA / ~120 V). (Don's number may have been figured on a larger current).
The parallel resistance for a standing human (nominal 1000 ohms) plus the copper is 1/((1/0.015)+(1/1000)), which is 0.0149998 ohm.
The current that flows is inversely proportional to the resistance of each parallel path.
The human to the copper ratio is 1000 ohms / 0.015 ohms = 66667, meaning that 66667 times as much current flows through the copper as through the human. 1 A / 66667 = 0.000015 A through the human.
I have done this in rather round numbers.
More exact calculations would require the resistance of both halves of the circuit with one including the bulb, the resistance of the human pathway back to the circuit source, and the exact voltage at the circuit source.
The problem I with Don's calculation using a 1.8 V drop is that it does not invoke the parallel resistance formula. If I provide a parallel path with a resistance of 0.015 ohms, his formula yields an amperage of 120... and you know it's not a 14500 W bulb.
