Releif u/s 89

1. You need to compute your tax liability 4 times for the 2 AY pertaining. 
2. Compute tax liability including arrears salary (including surcharge) and tax liability excluding arrears. Determine the difference between these liabilities and consider that as A 
3. Compute tax liability including arrears (including surcharge) in the year in which arrears relates and tax liability excluding arrears. Determine the difference between these liabilities and consider that as B 
4. If A exceeds B then you will be eligible for relief u/s 89. 
5. Remember, surcharge wherever applicable on the above income needs to be applied accordingly. 
please correct me if the above solution has an alternative view.