A brake lathe has a tool bit on both sides of the disc and cuts both sides at the same time to ensure both sides are parallel...that's the most important part of the whole deal
you don't even need to have it perfectly concentric to the bore as the pads don't cover then entire rotor surface, but if they are not parallel you will feel it in the pedal, as the disc gets wider and skinnier it will pulsate in the pedal as the hydraulic pressure changes...even squareness is not massively critical as the whole caliper assembly is floating
Brake lathes are pretty simple deals...but they cut both sides at the same time....so if you can be sure to cut both sides perfectly parallel then you're golden