Some problem regarding axis maintenance

I got a little layer shift (about 0.5 mm) on my print. So, I follow the tutorials. I got some problem in this step: My print is M300 Dual.

  1. It is difficult to balance the tension between the upper and lower part of the belt, after pushing the block. I use the tape and it works when the difference is large. However finally there is always some tension difference hard to eliminate. Is there any other method to balance the upper and lower belts except using the tape? This one would be the final solution for me. Belts Replacement in M Series 3D Printers | Zortrax Support Center

  2. There is always a really small gap existing, even if I adjust the tension of the belt to make it more narrow. To ensure there is enough tension on the belt, I could not adjust the belt too much, which means the gap could not be eliminated. From the saleman/technician who sold me the printer, he told me the little gap is not so important.
    So my question is, what is the bearable width of the gap between the jigs and the axis? Or it is strict that no gap should exist. In this case I may consider return the printer to professionals, which may take a very long time.