Why do plastic rulers make you measure between two lines, instead of starting at the edge?
tl;dr: Never use the edge of a ruler or the "zero" of any analog measuring device when you are making careful measurements if you can help it. Instead try measuring between two equivalent demarkations.
from this answer; doesn't start moving ...
Converting between different frames of reference is pretty straight forward.
You subtract the velocities. That's it.
If you have the spacecraft's velocity relative to the Earth, you can just subtract the Moon's velocity relative to Earth. Then you have the spacecraft's velocity relative to the Moon.
In the case of a lunar transfer orbit, the spacecraft is ...