Back when AK-Builder flats didn’t have the top rails cut to size, we had to do the work.  I’m talking circa 2006 and then late that year he came out with a simple layout tool that was so awesome.  You applied Blue Dychem do your flat, clamped the layout jig to the rail, used a scribe to mark the profile and then you came through with your mill (or Dremel) and cut out the shape.  It was faster than doing it all by hand and the one plus was that you could get the exact spacing between the upper rails that you wanted.