Even with nice smooth even brake wear…the THICKNESS is the key. With the rotor off the car there is a minimum measurement cast right into the steel on the back. If it were grooved , you’d measure the groove that cuts down the most , ie the thinnest part.
– one thing the all manufactures have done in recent years to lighten up the overall weight of vehicles is to make the rotors damn near minimum at the very start which is why you see SO many rotors needing replaced at the first brake job. ( if you were to lathe off any steel at all , it would be below specs. )