In these wonderful days of FMCs, you can project a straight slope back from the screen height at the threshold to stay above all the step-down fixes and convert the MDA to a DH -- just make sure you key it in right and monitor those step-down fixes.
The first instrument approaches were non-precision approaches. NDBs were new fangled technology and the criteria were designed around the DC-3 -- and still are except that the circling minima were given widened bands as approach speeds increased.
On my very first IFR flight test, as the stopwatch ran out the time on an NDB approach, I lifted my hood at the MAP to find the runway half a mile off to my left after what looked to me like a good tracking of the course -- and passed! I'd really want to know very well the guy who swung the compass before doing an NDB for real -- today you take along a GPS.