Clarification for page 11 in the manual of the XRAY X4'26:

Ball studs for the hubs. Rear initial setting is 0.5mm underneath the hub. This will, in combination with the longer 6.5mm ball stud, have minimal clearance for the CVD. And thus, we recommend using 1.0mm of shim, for sufficient clearance.

Keep in mind that:
1. The hub heights are -0.5mm compared to X4'25, so typically you'll want to run them higher anyway.
2. And - especially on carpet, we do prefer a higher rear hub height, especially in spec classes.

Using 0.5mm will have approx. 0.3mm of clearance, which is possible to use, but if you definitely want to use the 0.5mm of shim, and with bigger clearance, there's solutions:
1. Use the #303262 5.5mm ball studs (will have to be bought separately) also for the rear arms
2. Cut the ball stud shorter with a dremel tool