There are few steps to consider in order to get the proper lineweight set in drawing plotted correctly.
First you must consider which plot style to use, namely, color (.CTB) or named plot style (.STB).
In color plot style, AutoCAD will translate each color to a lineweight as set in the color plot style table (.CTB).
If the drawing use color plot style, just assign the layer to a color, say Red (could be any color), and then edit the Red color in the .CTB table and set it to 0.30mm. Downside of this method is that everything that is Red in your drawing will be plotted in 0.30mm. Alternatively you can set the Red in the .CTB table lineweight to "Use Object Entities". This way the AutoCAD will use the lineweight assigned to the layer itself - which is 0.30mm as you have set in drawing.
Color plot style is simple to use but will limit you when it came to assigning color to each layer or object. If say you already set Red as 0.30mm then you can not use Red for other object/layers that are intended to be plotted in other lineweight.
In named plot style (.STB), you create a named style rather than setting color to represent what will be plotted. This method will give you "freedom" in assigning color to objects and layers. Say you create a new style named Medium and set its lineweight to 0.30mm. In layer manager, assign all layers that is intended to plot in 0.30mm to Medium style. In this method, the layer will always be plotted in 0.30mm regardless of its color assignment.
Last advice - To really understand AutoCAD plotting system is not something that can be done overnight. Hope you take some times to play around with it.
