One employee in my office joined in service in January, 1983. G.I subscription started as per GISS, 1983. He opted to convert into GISS 1987. How to calculate  the final amount of GISS, 1983 for the period from January, 1983 to Oct, 1987?

online GISS 1983 tables available for more than 300 months. if anyone share the table for the period from 1 month to 300 months, it will be helpful.

Thanks in advance.