plot hamming distance with threshold

I have a verification system , my dataset contain 25 person each one have 8 samples

I found inter_class (size: 1*600 value:0.079 to 1) and intra_class ( size: 1*175 values:0 to 0.6772) using hamming distance between binary vector to examine performance of my system

I want to plot them in Matlab and having like this figure:

I want to use hamming distance with threshold

Did I draw plot then find threshold value from them or choose threshold value then draw plot? how to do it?

Then who to find False acceptance (FAR) rate and False rejection rate(FRR)?