The MPCNC has the entire weight of the Z axis motor and stage resting on the leadscrew, so the instructions call for preloading the spring coupler by stretching it with the leadscrew butted against the motor shaft. The leadscrew end isn’t particularly flat, so I inserted a 1/4 inch ball bearing between the two before the stretch:
I’m reasonably sure the ball won’t make the slightest difference, but two slightly misaligned shafts can now pivot on a point, rather than grind against each other. There’s no evidence of misalignment; I feel better and that’s what counts.
The Smell of Molten Projects in the Morning 
We used to do this exact thing at a company I used to work for. I would also say make sure the leadscrew is only in as far as the solid part of the coupler, let the motor shaft go in further. If you sink the Lead screw all the way in the coupler can’t “wiggle” that’s technical for “do it’s job”, hahaha. Loving the articles, I have a ton of reading to do.
I originally put it together with the 8 mm leadscrew shoved all the way up against the 5 mm bore on the other end, then noticed the coupler couldn’t flex. So I pulled the leadscrew back to the solid part, extended the 5 mm motor shaft to meet it, then noticed the shafts would grind together. Just takes me a while, some days.
And … welcome aboard!