Thanks for the vote of confidence on all my creations!
There is a cut calculator linked at the top of this page, https://www.v1engineering.com/lowrider-cnc/
Y rails are up to interpretation. The main thing is usually people use the large machine to cut all the way through sheet material. If that is your main use, the Y rails do not need to be very accurate. If you have another use you might want to pay more attention to how accurate you can make it. I honestly just slapped mine together on the flattest surface I could rig up just for the build, and it actually came out pretty good after screwing on a top and bottom surface.