Relief U\S 89 can be computed as follows:-
1- calculate the tax on the arrears received by applying the rates applicable in the AY in which it is received.
2- then calculate the tax on the income by apllying the rates which was prevailing for the years for which the income is received.(here you have got arrears from 95-96, so you will have to do the calculation for everyyear since 95 by applying the rates and taking the income accrued for the respective years)
3- difference of 1-2 would be the releif yhou would get.
Note:- while calculating the tax for earlier years education and higher seco. education cess and surcharge would be calculated.