Wire Gauge Required Question

For all practical purposes the resistance of the wire does not change although it will change slightly with temperature. So the voltage drop is a linear function of the current. You can make the percentage drop a percentage of anything you want. If you want to know the voltage drop in the wire as a percentage of the voltage between the panel and the battery you would have to use the panel voltage less the battery voltage. That would only give the percentage drop due to the wire. There are other losses. In addition, the controller and battery determine, to a large extent, the current into the battery. Hope this makes sense.