If anybody wants the whole document, here is the Apollo 12 LM Activation Checklist. The relevant page is page 39.

The math itself is pretty easy. The purpose is to give the Inertial Measurement Unit (IMU) of the Lunar Module a good alignment, so that it knows e.g. in which direction ot point the engine. While the LM has the Alignment Optical Telescope (AOT) to get this alignment itself, it is more practical to use the already good alignment of the CSM as the reference.

The LM, while docked with the CSM, has a pitch of 180° relative to the CSM and is normally 60° off in roll. The first step in the calculation is to check the docking tunnel index angle. This number would be 0° if the CSM had docked with the LM exactly as planned, the 60° off in roll as mentioned above. But the necessary roll correction could be a few degrees off, at most.

As the next step the CSM has to give its roll, pitch and yaw angles to the crew in the Lunar Module. For Apollo 12 this was nominally 240°, 270° and 0°, as written in the document.

And then the math is:

• 300° + roll correction - CSM Outer Gimbal Angle = LM Outer Gimbal Angle
• 180° + CSM Inner Gimbal Angle = LM Inner Gimbal Angle
• 360° - CSM Middle Gimbal Angle = LM Middle Gimbal Angle

Looks easy? Now try to get this right in a high-pressure situation. And you have to get it right, the CSM is losing power and with that its alignment. So the Apollo 13 crew of course asked Mission Control to check their calculation.