If the top is ahead of the lower ball joint Caster is negative . Of course if the top is even with the lower it will be the 0 mark shown above. The proper perspective here is that we are looking down at the front of the car. If the leading edges were turned away it would be negative Toe . If both tires were perfectly parallel with the vehicle center line they would be at 0 Toe.
Caster has little to do with any form of tire wear.
Toe is the Godzilla of alignment angles. Check your tires for wear each time you rotate them to stay ahead of possible Toe wear. This type of wear can be easily missed when it is on the inside of the tire. Here is a separation in the tread itself. This will cause a wobble effect at lower speeds and a pull to one side.