It is quite difficult to make a constant-radius corner in BTB if you rely solely on judgment. The 2D view shows the track in wireframe mode, and lots of lines at different angles can create optical illusions. You can make the process much more accurate by using a picture of a circle as a guide.
Create a square image and draw a circle on it. It helps if the circle is drawn using the 'antialias' option to give it smoother edges, and it should be a few pixels thick (5-10 is fine):
(Instead of creating your own image, feel free so save the above image to your computer)
In BTB, load this image as a background image:
You can move the background image by dragging the centre point. You can resize it by holding Ctrl and dragging the corner points (holding Ctrl maintains the correct apsect ratio). Move and resize it until it lines up with the start and the end of the corner:
You will now be able to see the the difference between a genuine circle and the shape of your corner. When modifying the corner, it's best to have a node at the start, another one at the end, and none in between. Then you can move the two control points (circled in blue) until the track lines up with the image:
Sometimes it may prove very hard to get the track to line up properly. No matter how you adjust the control points, some parts of the corner may be too tight and others too open. In this case, adding another node to the track (Crtl click) will make it easier to get the right shape.