what is C?
C is total (analytical) concentration of acid. During titration acid will be present in neutralized and undissociated forms, sum of their concentrations will be constant (if you neglect dilution).
When you have added 31.64 mL you have neutralized 31.64/55.48=57% of acid. That means 43% (1-31.64/55.48) is still in HA form. These values should be put into Henderson-Hasselbalch equation.
should it be:
pH = pKa + log[A-]/[HA] ?
It is exactly the same, just solved for pKa (unless there is some error in my math). It can be expressed in many ways, like:
pH = pKa + log (57%/43%)
As you see the difference between your equation and the above is that you forgot about 43% - concentration of undissociated (not neutralized) acid.
You can afford to use percent under logarithm because once again everything cancels out. Only ratio is important.