I spent at least half a day adjusting my belt tension to try and get rid of backlash, then the belt broke. When I was changing the belt I noticed that the pulleys were not tight on the servo motor. A lesson learnt for me at least. I was led to believe If the lip is at the top of the circle it is probably the y axis and at the side the x-axis
In the end I settled for the principle when I pluck it the belt should stop vibrating in approx 6 seconds. As others have said not too tight as this can lead to strain on the motors and mechanics. Also I am wondering about the quality of some of the belts out there I bet they are not all the same.