I'm also wanting a 12-28, but with closer midrange ratios 12-13-14-15-16-17-18-19-21-24-28.
I intend to modify a 12-25 by adding part # Y1RK98030 which is the 21-24-28 sprocket unit from a CS-M8000 11-42 cassette.
While this is a MTB cassette I note that it uses the same 2.18mm spacers as road. It may need some machining down of the spider on the spoke side.
Another option would be to add the 22-25-28 from an SRAM 11-28 cassette. While this has the same 19-22 jump, I'd prefer the more linear 19-21 as per the above.
Last edited by nz_aj; 10-03-16 at 01:58 AM.