You can use this method dear :
Basic= 40% of the CTC
HRA=40-50%(It depend whether the person put up in metro or non metro city)
Conv.all=800 (But conveyance cannot be more than Rs.800 because even 1 Rs. above it will make it taxable)
Spl.all= Balancing factor (This will be the overall remaining amount that you can put after all calculations)
A) Total= Sum of the above
PF=12% of the basic salary
ESI=1.75% (If applicable)
LTA/Medical= 8.33% of the Basic or one month Basic i.e. Basic/12 (But medical cannot be more than Rs.1250 because even 1 Rs. above it will make it taxable)
Bonus= Rs.700 (if applicable)
Gratuity= 4.81% of the basic
B) Total= Sum of the above
Total Monthly CTC= A+B
Annual CTC= (A+B)*12
In case you have any doubt just let me know.
Regards
Saurav Verma
Head HR- MB India