There was a recent thread on this on the frameforum. A simple way to go is to get a wheel, mount the caliper on your bracket, and clamp the caliper to the disc (try to do this so it is in mid adjustment), then mark fit and weld that place. There are standards, however, and I think those were discused in the other thread.
I found the previous thread. Since I'm building a new bike from scratch, I will have to get my caliper and measure a bike at the local shop. A search for info on the ISM (International Standard Mount) got me a lot of info on calipers, but not much on the mounts, other than that they are 51mm apart.
Select "Downloads" from the "Service" manual that appears at the top of the Magura page. Select "2007", then "Disc Brakes", then "Marta". The last link on the page that appears should be "Tech Sketch: The Dimensions of the International Standard". It's a PDF containing exactly the info you need. Cool find!
Hint: the drawing labeled "Rearbrake" is actually a front fork, and the drawing labeled "Frontbrake" is actually a rear drop-out.