Thanks for the correction re. Tan vs Sin. Tan is obviously slant range and Sin is actual surface distance... For small angles these work out to be practically equal... 317.99 and 318.42.
As for terrain separation, this could result in a non-standard glideslope angle.. 3.5 degrees for my local airfield, with some going to 5 degrees in more difficult regions.
From a practical point of view, and maybe a trick question... How long do you spend at each DME mark when traveling at over 120 knots?