CR-10 X Idler Shim

I had noticed that the x axis belt on my CR-10 does not run true - the main problem being the idler which is off centre forcing the belt to run more towards the back of the machine rather than through the centre of the x axis bar.
Looking closely as in the first picture this can be seen clearly - whilst the belt is up against the wall of the idler there is no play left to centre it.
So I decided to fix the issue by creating a shim to go between the idler bracket and the x axis bar - second pic