how can i find 20% or 80% of value in the decreasing slope by keeping maximum peak value of y axis as reference?

how can i find 20% or 80% of value in the decreasing slope by keeping maximum peak value of y axis as reference? for the mentioned type of image

[Birdman]

Firstly, are you interested in intervals with negative slope?

Secondly, you just want to find %20 or %80 of the values in the intervals with negative slope?

[yoga lakshmi]

i want to find 20% or 80% values in the interval with the slope with peak amplitude value as reference

[Birdman]

How did you obtain the data? Can you share it so I can be more helpful.

[yoga lakshmi]

clc; clear all; close ; x=dlmread('D:\iprmay\26378D.txt'); t1=x(:,11); a1=x(:,12); figure,subplot(6,1,1);plot(t1,a1);title('Shot 26378D plasma current'); this is the plotting i used.since the file is too large couldn't able to attach here.the a1 and t1 are numerical data values.