While calculating Gratuity I have few queries Please help me resolve the same

1. Do we need to use total earned basic till last served month or it is a fixed Basic as per his/her Latest CTC?
2. Why do we need to multiply per day rate of gratuity with 15 days & then year? I mean why 15 days? what is the logic behind?

For Gratuity, you have to follow this logic
If P is the number of years of service in a company, Q is the last drawn salary [i.e Basic Salary+ Dearness Allowance], then,
Gratuity = P x Q x 15/26 ; 15 being wages for 15 days and 26 being the days of the month.

Consultme,
You are not aware of the calculations. If we calculate gratuity as per your formula, employee will receive gratuity at quotient 15/26 which actually works out to half entitlement. The formula is:
P*Q*15 days*30/26.
15 days means half month as per the act; there is question of logic in a law.

As per Payment of Gratuity Act 1972, Wages are defined as all emoluments which are earned by an employee while on duty or on leave in accordance with the terms and conditions of his employment and which are paid or are payable to him in cash and includes dearness allowance but does not include any bonus, commission, house rent allowance, overtime wages and any other allowance. So practically it is determined as sum of basic pay and DA. CTC is wider and includes amounts which are actually not paid to the employee (like cash value of leave employer's contribution to PF, ESI, Gratuity etc) . So, that cannot be used.
As regards calculation of Gratuity, the legal provision is "For every completed year of service or part thereof in excess of six months, the employer shall pay gratuity to an employee at the rate of fifteen days' wages based on the rate of wages last drawn by the employee concerned". Thus it has to be monthly wages divided by 26 and then multiplied by 15. There is no need to bring in 30 in the calculation. The formula shall be P*(Q/26)*15

Regarding the first question of the post, I think that the answers already given by our learned members are quite suffice and need no more elaboration.Still, the only point that I would like to emphasize is that the poster should make a thorough reading of the existing threads in the Web pertaining to the concept of CTC and have the right understanding that CTC is just an accounting tool to assess the overall annual expenses incurred per employee from only the employer's perspective and has nothing to do with the actual statutory pay-outs to the employee.
Now, as regards the second question,Section 4(2) of the PG Act,1972 lays down that for every year of completed service or part thereof in excess of 6 months, the employer should pay gratuity to an employee @ 15 days wages based on the rate of wages last drawn by the employee concerned. That could be the simplest answer to the question why 15 days. If we go in deep for the logic or reason if any behind this statutory dictat, we should accept that wage is the consideration for the work done by the employee and it can be in cash or kind or both and again the measurement of the work can be in terms of quantity of work done or in terms of the time spent by the employee to do the work. Again time can be measured in terms of hours, days, months or year. What would be the most convenient unit of measurement of time for any work done by an employee would be "the day" only in respect of the modes of wage payment contemplated in the first and second proviso of sec.4(2) of the Act.