Get X value which have 90% area at specific TF1

Hello rooters,

i apologize upload this weird question.
But I need a comment on this issue.
I want to get x value at specific TF1.
My function is,

double xmin = 6e25;
double xmax = 1.6e26;
TF1 * pdf = new TF1(“pdf”,"[0]*Poisson( ( [1]/x ), [2]) / Power( 1/x , 2)", xmin, xmax) ;
pdf->SetParameters(1, flux(energy), event(energy));

At this function, I want to estimate x value which is 90% weight have (when integration between xmin to this x)
Seeing Below figure is much better to understand easily.

I couldn’t solve this issue using my skill.

Thanks, guys.

ROOT Version: 6.08.04
Platform: Ubuntu
Compiler: Not Provided


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