If you look at the lecture slides on page 9, you'll see how you can pick off P_Q with a 95% confidence interval from your load vs. COD plots. This P_Q value is the critical force you need to plug into the K_Q equation.

I'm not exactly sure what you mean by critical stress...there's no stress in the K_Q equation. If you are referring to the stress term in meeting the conditions for K_Ic, that is the yield stress of the aluminum given as a 0.2% offset as specified by the ASTM E399 standard.