sepia-tone settings

I have written a program to set the sepia-tone using the convert  
function. Program works fine but I am not able to get a desired  
effect. I am trying to achieve the sepia settings one would get by the  
default PS3 sepia setting action. In my program I have tried threshold  
70% to 99% but none matches the effect I got by running the sepia  
action in PS3. I can send the sample image of the PS3 converted image  
and the programatically converted image if anybody is interested in  
helping. Thank you.