The percentage error in a measurement is calculated by taking the absolute difference between the measured value and the true value, dividing by the true value, and then multiplying by 100 to express it as a percentage. 

In this case, the measured value is 255cm and the true value is 250cm. 

So, the percentage error is:
\(\frac{255 - 250}{ 250} \times 100 = (5 / 250) \times 100 = 2%\).