I am giving u the case where assessee is calculating interest u/s 234B. Section 234B is applicable
if tax paid is less than 90% of returned tax.
90% of tax 21,212 comes to Rs. 19,090. Tax paid is 18,000. Now tax paid is less than 18,000.
Interest u/s 234B shall be calculated on 21,212 - 18,000 = 3,212. Round off to 3,200.
Period of default shall start from 1-4-2011 till the tax is paid.
Now suppose tax is paid on 31-7-2011. Period of default is 4 months.
Interest 1% of 3,200 x 4 = 128.
As when your tax increased in the assessment interest shall also be increased.