Find optimal quantum at Round Robin Scheduling algorithm In this process list situation

How can I find the quantum time ( time slice ) at 80% of CPU time ?
I found only for 100% of CPU time, so I don't know if it's the same for 80% case

B.T.Q= [mean + median ] / 2
mean=(P1+P2+P3+.....Pn) / n
Never mind, I just found how to take the quantum time. Just find 80% of n processes Eg: You have 9 process, 80% from 9 is 7.2 ~ 7. Order the process ascending by BURST and take the burst from 7 process.