Gratuity Calculation ?????????/ - Xls Download

Respected seniors,
I need your help as my co. outsource data entry operator, I T support employees etc on contractual basis.
Individual has to work on a particular customer site on behalf of my co. As per the contract our customer is paying gratuity on monthly basis.
Is there any specific rule or procedure that you have to record the gratuity on monthly basis / on days basis?
Is there any legal format which you have to maintain at the co. end?
As per my knowledge gratuity is applicable after 5 years of continous services and calculation part consists (15 days of last basic salary X no. of years performed)
Please do the needful ASAP ASAP ASAP ASAP ASAP ASAP ASAP
Regards
Vikas Kumar

Dear Vikas,
I take any company will calculate your Gratuity from day one of your service, because Gratuity is part of your CTC. But when it comes to paying of Gratuity i am so sure of this.
According to me legally Gratuity can be Paid on completion of 4 Years and 240 days of service in any company. Correct me if i am wrong. By the way i am attaching a excel conatining examples of Gratuity calculation which might be of some help to any one of you.
Regards
Amith R.