Since you didn't give the actual volume (or any of the experimental values) I can only tell you how to do it. Do the calculation using the real (determined) volume of the flask. Then, re-do the calculation with v = 125ml. Take the two values and calculate % error; m = measured vol; g = guessed vol.
[mW (m) - mW (g)]/mW (m) x 100%
(they want % error so, if it is negative, just get rid of the sign)