Ah PC,
When I went to school in the very early 60's, 135mm from 150mm was 15mm/.590" difference, divided by 2 = 7.5mm/.283" + or - approximately.
How did you come up with 12.5mm per side?
The brake side will be less than the non brake side...at least from my quick measurements, to keep the brake centered between the hydro brakes.
Or are we talking about something else/different ???
JJOYC = Just jerkin on your chain...lol