Mirror line

I dont know if this is asking for a plug-in or if this feature exists and i cant find it.

I want to be able to mirror an about along a specified line. The problem with the mirror currently in blender is that it pics where to mirror the object and usually i want some certain point to line up in my model so i can put the two sides back together. I currently have a workaround but its long. Anyway ill explain it here so you can understand what im getting at.

First i put my cursor on a vertex along the desired mirror line.
Next i exit edit mode and set the center of the object to the cursor
Then Duplicate the object and rotate it 180 degrees about the center (although if the left and right sides are different i have a problem at this point.
Then i join the meshes back together and eliminate doubles.

Is there an easier way?

Not too sure if I understand you correctly but I think you mean the mirror modifier
If this is not correct please post a screen shot or blend.
hope this helps

IF you whant, you may mirrow around cursor - it’s give you more freedom imho.
In any case don’t understand you problem)

its the mirror modifier thing