Just sitting here at computer, I'm not sure if this will work or not...but, I would think, geometrically, as the trailer reached the "touchy" spot, the same landmark on trailer would be nearing outside edge of your side mirror. In other words, as you back and turn tighter the trailer become more perpendicular to truck and parts of the trailer would be appear to be going "off" the outside edge of mirror as they are further from center-line of truck.
Have a spotter stop you shy of actual contact (leaving some safety factor) and look for a spot on your trailer at very edge of mirror you can use as gauge when backing alone. You could do think in large parking lot pulling straight and testing the spots (one for each side) you picked. You would need to be pretty consistent on your seating position, too.
Or, this might be just some left-field thoughts :)
