Dear Mahal,
The Gratuity calculation is depends upon your company's policy. Your company already made tie up with LIC you have to caluculate according to the guidelines of the policy.
The second one is according to The Payment of Gratuity act 1972. i.e.
Wage last drawn (Basic+DA)/26*15*no.of years of service rendered by employee.
The minimum service for caluculation of gratuity is 5 years and it is not mandatory in cases of death.
In above formula 15 means 15 days, if the company is interested to give beyond 15 days that is their wish.