How do I do a straight cut of all 4 legs of a metal chair?
Woodworker's solution: put chair on flat surface. Put shims under feet as needed to level and stabilize it. Using a pencil on top of a suitable scrap of wood, mark all sides of all four legs at the same height above the table. Cut to these lines. If necessary, fine-tune with sandpaper, but generally this will be close enough that flexibility in the materials will do the rest.