Your usage of units doesn't look too good. Did you really start with 250 cm3 of NaHSO4? And then titrate the lot of it? What does the mean titre refer to? The NaHSO4?
Ohhhh if course, I used 250cm3 of a NaHSO4 but I need to use my mean titrate as the volume for the NaHSO4 in the calculation don’t I ? As the 26.7 was the amount of volume required to neutralise the 25cm3 of NaOH...