How do I control the line thickness that is assigned to a particular line weight while plotting? For example, I select '6' as a cut thickness and I want it to be .5mm in line.
These are defined in the print/plot drivers.
In the Print dialog please go to the File menu > Edit Printer Driver Configuration.
Look at the Weight Maps tab.
There is information on this in the help documentation under the heading "Printer Driver Configuration Dialog, Weight Maps Tab".
In a system where the print drivers are shared, edits will affect other users and may need administrator access.