The easiest way to track wire or string is to make sure that you convert it to the same units that you would be using when you create a piece.
ie if you buy wire in meter rolls but use it in centimeter lengths then convert it to centimeters BEFORE adding it to your parts inventory.
So for meters just multiply the number of meters you have by 100, so 3 meters = 300 centimeters.
For feet just multiply by 12 so 3 feet = 36 inches
Then, using the examples above just enter in to your parts inventory a new piece as follows (please use the correct numbers for your lengths).
For the meter roll;
# Pack as 300
Units as 1
For the feet roll;
# Pack as 36
Units as 1