Question: how to calculate percent error of length?
Answer: Subtract one value from the other: 2.68 - 2.70 = -0.02.
Depending on what you need, you may discard any negative sign (take the absolute value): 0.02. ... 
Divide the error by the true value:0.02/2.70 = 0.0074074.
Multiply this value by 100% to obtain the percent error: