Gratuity calculation rules - company should calculate from the last drawn salary?

My organisation calculates the gratuity every year at the start of the FY with the salary revision.
But as per rules, company should calculate from the last drawn salary?
This will make a difference of final amount if they calculate every year and not by calculating the finally.
Please confirm.

in our organaisation we are calculate the gratuity amount only for last drawn salary.