Fatigue crack propagation

Hi, I'm trying to perform a supposedly-simple experiment on fatigue crack propagation of UHMWPE. I use a compact-tension specimen on a servopulser hidraulic testing machine. I will try to do the experiment on a fix Pmax with a varied force ratio (R) from 0.1 to 0.5. I'm having trouble in deciding the maximum force Pmax. How does one decide the amount of force applied for FCP testing? Calculation steps and so on. I have read so many papers and journals I could get my hands on, including the ASTM E647 (Standard Test Method for Measurement of Fatigue Crack Growth Rates), but none have mentioned on how to decide the amount of force applied. 

I would be so glad if you could help me. Thank you in advance for your help.