how to calucate the Gratuity ?

Hello Members and admins,

i m new for this forum.
i m working as a admin in a private company. we calucating Gratuity money for our staffs by manually, but its tough to calucate. can you help me to put a formula for calucate Gratuity of our staffs?

Gratuity Details:-
if experince 1- 5 years Single basic for a year
if experince 5-10 years (1st 5 years 5 basic salary for a year and next 5 years 1.5 of basic for a year)
if experince more than 10 years (ist 5 years 5 basic salary+ next 5 years 1.5 basic salary for a year+ after 10 years experince it will 2 of basic salary)

example in attached file

staff no 101 has experince total 13.22 years.

so first 1st 5 years he should get one basic salary for every year as Gratuity (4000*5)=20000
next 5 years he should get 1.5 basic salary for every year (4000*5)(1.5)= 30000
next 3.22 years he should get 2.0 basic salary for every year 4000*3.2)(2.0)= 25760
total Gratuity 75760

plz help me to calucate this by a formula.