I spent some time looking into it and found a pretty nice solution.
When I made a new buffer layer in MMQGIS it's making a new layer with an extra area respectively to the radius we defined.
So, if we have only the outer line of the buffer we will get the borderline we needed.
In the Vector tool bar there is an option to transfer a polygon layer into a line layer that represented only the outer line of the polygon
We need to be sure that we are choosing the right layer and some other layer from our project
Once we have done thus steps we will have a temporary layer which is recommended to save as a layer.
It's also recommended to measuring the radius, just for being sure that our result makes sense
We cannot be one hundred accurate in this kind of hand measurement, so small a deviation of 880 meters, or 0.58%, it is probably as good as it gets.