Really..c'mon now...it was an honest question.
What is the altitude at the DME rings based on.
You will NEVER get the same answer as on the charts by simply using tan 3 degrees.
As you all are aware, the closer one gets to the DME, the less accurate it is.
Its not until you are about 2nm that the errors balance out..fly 3 degrees parallel, and well...you get the picture (I hope) I wont even mention geologic conditions.. One variable that is not in the equation, obstacles, the DME ring is not an obstacle clearance surface.
Safe to say, as I and others have noted, it is a voodoo mix of many varibles that the State source uses that result in the altitudes you see on the charts, much like the MSA circle...