* is the rotor true? It only takes a slight warp in the rotor for this to become an impossible task (but still perfectly rideable). Don’t assume a new rotor is true, I’ve had to send 2 in a row back to the online shop I bought them from.
* save your sanity and know when good enough is good enough – unless all the parts are 100% new and bang on, the set up never will be, end even then it’ll start to degrade a tad with riding.
It’s a bit of a judgement call as this is the interface between theoretical in the lab engineering, and the crufty jaggy entropy-dominated real world.
The acid test is, with decent power in the brakes and the pads pumped out, you can spin the wheels with no drag and the braking is smooth.