Potentiometric Titration Lab Help

Hello,

I am working on a potentiometric titration lab for a diprotic phosphoric acid. Attached is the excel document of my lab. 

We are diluting 1.022 M NaOH by a factor of 5 and then using it to titrate with 25 ml 0.06M Phosphoric acid in a beaker. 

I know my titration curve is correct, however my TA told me that the scaling of my x-axis is wrong for my first and second derivative curves. I know that I need to use the average volume. I also know that I take must take the raw volumes and correct for the initial reading in order to find the average, but I am not exactly where I am going wrong. 

What I am doing is taking the second volume and adding it to the first and dividing it by 2.

Such that it looks like, (as taken from the spreadsheet):

           (A4+A5)/2    or     (3.88+6.3)/2

If someone would be willing to help me out that would be great!

Potentiometric Titration.xlsx