Did you get everything from here? Did you modify anything?
Scaling of just the roller F should be okay but you really should not have had to scale it down most people are scaling it up. What size conduit are you using?
Both of mine cut great circles. If yours is sitting cockeyed before it starts I suggest loosening all the roller locks and all the center bolts and all the z axis stuff while it is assembled and try to tighten it up a little at a time seeing if anything is making it crooked. If you look at the parts they should really be trying to pull it in to a nice square. If you assemble the center real tight and put it together you could have actually assembled that crooked. That is why the directions say not to tighten anything until you are ready to run it.
Let me know.
People have done some other fixes by aligning the axis every time before they start a cut but really it’s easier just to put in the work once and never have to touch it again.