Could the assumption for calculating how many years to apply the inflation be as follows: Assume that the average start date for reinsurance contracts written on 1 January 2012 is halfway through the year i.e. 1 June 2012 so the inflation period would be from 1 June 2012 to 1 October 2015 which is 3 years and 5/12 months.
ILF table is for Reinsurance contracts written on 1st Jan 2012, regardless of mid-point of risk. Actuary wish ILF curve for Reinsurance contract that will commence on 1st Oct 2015, regardless of mid-point of risk. Now, if you assume mid-point of risk spread is after 5 months(1 Jan 2012 to 1 June 2012) from commencing, you should also assume mid-point of risk spread is after 5 months for the contract that will be written on 1 Oct 2015. just needs 1 assumption that - risk spread is the same for Reinsurance contracts wrote on 1st Jan 2012 and 1st October 2015, and theirs Reinsurance contracts risk also remain same.