As we know, the Malus's law is:
Presently, I have a polarizer system (one half wave plate in front of a polarizer) in the lab, and have a powermeter. So, I can get two key factors: the input intensity of $I_0$ and rotated angle $\theta$. However, in real environment, the adjusted Malus's law will be:
In above equation, I is output intensity, $I_0$ is input intensity, $\theta_0$ is offset angle, $I_1$ is offset intensity.
Here, my question is: How can I fix the value of $I_1$ by testing?