The machine it's self doesn't need to be level it could be at 45deg and workfine, it needs to be sat on flat surface with no twist.
It's getting the parallel rails level on the same plane thats the more important thing.!! . . . . Very hard to do and time consuming best done with precision straight edges and machine levels.
Even then it's not fun.!!! . . . Must have took me 6 weeks to tweak my machine into a position where I was happy with it.