How much self assessment tax should be paid?

Suppose total tax payable is Rs 20123 which is rounded off to Rs20120 and TDS is Rs19448. In this case should the self assessment tax be Rs(20120-19448)=Rs672 or should it be rounded off to Rs670 (or to Rs680)? If I pay 670 probably in ITR -1 form, in section tax paid and verification, there be Rs2 as tax payable and if I pay tax Rs680, it will show refund for Rs8. What should I do here? Second, in TDS tab, in ITR-1, item 25 contains both info for advance tax and self assessment tax. I am putting figure to mean self assessment tax but on next page, (verification) it is being shown as advance tax rather than self assessment tax. Why is this so? In fact how does excel distinguish between self assessment tax and advance tax in same table in TDS section?