Robert:

>
ABS(12-14) = 2, but what percent is 2 of the two numbers?
>

What is your context?

In general the percentage is measure as follows:
100% * ( Final Value - Original Value ) / Original Value
A positive value represents an increase in value.
A negative value represents a decrease in value.
You do have to check that Original Value is not 0.

There may be another way to calculate the percentage in some specific context and I do believe I heard about one when I was taking a class in Game Theory, but these are for very specialized situations.

Daniel