This file will help you find the K factor in your press break. 
Finding K factor , bend deduction and bend allowance is very important for a sheet metal work shop.
All you have to do is get a sample flat size and then fold it as per your press break. After that you need to measure following data using vernier caliper and put all those details in following calculator.
Please note that for this calculator I have assumed that angle is a perfect 90 degree.