The bush should be flush to the frame of the mount. You can also say that the distance between the two sides of the mount and the bodywork should be the same .
Generally speaking, if the bush comes off the mount by any mm, it will add stress to the drivetrain.
When i had mine replaced, i had a new bush pressed in. It needs a hydraulic press to push it in and my mechanic later on, advised me to install short bolt in order to keep the bush from sliding out. The mount frame has a small recess where you can drill a small hose and screw in a small bolt.
I didnt listen to him, i installed the new bush and thought tha since the hydraulic press applies so much force to the bush in order to slide it in, there is small probability that the bush slides out again. It has been around 5 years since i did the mount and now the bush has started to slide out again ... i dont think it is more than 1mm but i now concude that the forces that the mount receives are massive.
Therefore, the next time i do again the upper mount i will install a tiny bolt/screw in order to keep the bush from sliding out.