Rails
Once you get ....one rail level and straight... for the other rail....
I made a simple hardwood "T square" for setting up /with a digital caliper taped to the far end/ low end of the T
A saw groove on the wood T then sits on the first/ level rail....
the "depth gauge" works as a "feeler gauge" to get the rails exactly parallel.
If the rails are parallel and straight, the y car about has to run true.
The decimal point seems to be the most important on the z axis... x & y not so much....
ShopBot... Where even the scraps and things you mess up and throw away are cool....