Yes, that's the normal length. This is what you need for regular concentricity measurements.
To measure axial concentricity you need a longer bar. I don't know if you can buy one. I would think ground bar would be fine but I can't find it. I did reach out the that outfit you flagged earlier but they just want to sell bar and don't want to tell me what level of precision they are. The email chain keeps growing.
A full scope machine shop can grind you one but probably for $$$.
I would think that linear guide bars would work, but I don't know that for sure.
I cut my own but I don't need them to be what you need because I have pretty good metrology capabilities here. You don't have that so you need a known good bar.
You have shown that your reamer shaft isn't up to the task. So we just need to find something that is. I'm pretty confident that the group here can do that.
In the meantime, keep looking!
