Claiming section 89 benefit for leave encashment during service

I wish to claim relief u/s 89 for leave encashment during service. Due to inclusion of lumpsum amount in my income for FY 2016-17, I fall in 30 % bracket while I paid tax @ 20 % in earlier years. How can I bifurcate the lumpsum amount to earlier years ? Is there any thumb rule like 1/3 in previous 3 years etc. ? Kindly guide.