The BMW Z3 Bentley Manual (page 340-14) say min. rotor thickness for the front rotors is 20.4mm and rear is 17.4mm. My factory original OEM rotors on a 2001 Z3 M Roadster are stamped min. thickness 26.4mm front and 18.4 mm rear. The front rotors measure 25.8mm with no significant wear ridge with 17k miles on the car. Which is correct for the min. thickness: the Bentley Manual or what is stamped on the rotors?
I would follow what is stamped on the rotors. The spec in the book is not very clearly labeled, as it is for the regular Z3 which uses a 300 x 22 mm front rotor. You M roadster uses a 315 x 28 mm front rotor, so the 26.4 min thickness makes sense. At 25.8 you are slightly under minimum thickness. You mat not have an issue now, however as the pads and rotors wear, the rotors could become very under sized by the end of this set of pads life. I would suggest replacing the rotors any time they are under sized or close to min thickness when replacing brake pads.