One value has to be chosen as the reference value. Let's say the two values are a and b, and a is the reference value, then the %ge difference is 100|a-b|/a where |a-b| is the absolute distance between the values (a positive number).
Example: if a=10 and b=12, the percentage difference is 100|10-12|/10=200/10=20%. b is 20% (of a) more than a. That is, b is 120% of a.
Example: if a=12 and b=10, b is 20% (of a) less than a. That is, b is 80% of a.