I've just done this job on my 500.
I trial assembled the steering head with the new bearings. Sure enough the top bearing was sitting up a few mm from home.
I measured the vertical free play in the shaft and added 6 mm as the amount the journal was lengthened.
If you've done this before you would be aware of the possible difficulty of getting the shaft, or shaft with lower triple clamp to run true for turning.
I had press out the shaft and use the 4 jaw rather than set the triple and shaft up to run true between centers.
Marcus