SLED just monthe before from the manufacturing date

Hi  SAP User ,

We are changing SLED data of FERT material form Day 'D' to Month 'M'  .

Our requirement  , SLED will just before one month from  the date of manufacturing date /month .

But SAP calculating :-

Total shelf life in material master      = 12 month

Period indicator in material master    = M (month)

Manufacturing date given during MB31 say 10.04.2016

SLED calculating through SAP =  10.04.2016 + 12 month = 04/2017

But our requirement , it should be 03/2017 instead of 04/2017 ( Just one month before from the manufacturing date )

Please guide me how to resolve the issue .

Regards,

Amit Biswas