I need to prepare 0.02 molar sodium phosphate buffer using Na2HPO4 (141.96) and NaH2Po4 (119.98). I am confused about following things in calculation using Henderson-Hasselbalch equation.

1. What is the pKa value to be considered? Is it pKa at µ=0 (7.21) or µ= 0.1 (6.71)?

2. I calculated using both of these values and compared my answers to the buffer calculators available online. But my answer is completely different from theirs after multiple tries (I have ensured that all parameters were set properly according to my requirement). Are these online calculators reliable?

3. Can HCl and NaOH be used for final pH adjustment?