How to find the threshold value from KNN classification

 Hi all,
  I want to ask that I have total 100 classes [1,2,3,4,5......100] One class having 12 samples from which 8  
 samples for training, like (100 * 8 = 800 samples for traning). Remaining 4 samples for testing,like  
 that(100 * 4=400 samples for testing. 
 Now, I need to find threshold values, which will assist me in identifying Threshold,FAR,FRR, by using  
 Matlab.  
 I have studied perfcurve function, but I don't know how can I measure the threshold values 100 classes. 

 Please guide me how can I do it?