dracker prager plasticity model with softening

Hi my friends,i am using Dracker prager plasticity model to simulate uniaxial compression (softening) of coal samples.in dracker prager, the shear criterion is linear, and the hardening type is compression. there is a table that i should fill out, this table composed of yield stress and abs. plastic stain. my question is:- how can i fill out this table by using the experimental data (stress-strain diagram for uniaxial compression test) ?? shall i convert these data using any equations?? or i will estimate the yield stress, ultimate stress and post peak stress and find the corresponding strain for each of these values and put these values directly in the table as it is??  note i want to simulate the softening of the coal sample. and i have the actual stress -strain data. best regards,Gamal