The total circuit length is a round trip so half the circuit length is the distance between the two items (panels, boxes, etc.)
To add to the complexity, there are different rules for wires hung in free air (such as overhead) versus wires in Romex cables or buried or in conduit. And a further calculation, called derating, for several circuits' worth or wires running juxtaposed. The rule of 14 gauge for up to 15 amps and 12 gauge for up to 20 amps is for wires within cables running singly.
(One morning on WRKO Radio) The tub's not clean! Let's remodel the bathroom.
(One afternoon on WRKO Radio) It's time for an oil change. I'm going to trade in the car for next year's model.