How to prepare a 0,2 M phosphate buffer (Na2HPO4-NaH2PO4), pH 6.4?

I'd like to prepare 0.2M solutions of Na2HPO4 and NaH2PO4 and mix the two solutions (I calculated the correct volume of each solution with the Henderson-Hasselback equation) to obtain the right pH. But I found some tables on internet in which the two volumes to mix were already calculated (but it is not written how) and they are very different from my results.
Anyone could help me please??