Having difficulty with the OP title there, Bob?
Well, if we're not happy talking about the shape and size of the earth, let's go back to N Korea's missile test and do some maths, shall we?
You seem reasonably happy that the reported range of the NK missile test, 950km, is accurate. You also seem content that it was indeed a ballistic missile, meaning that after a short burn its trajectory was entirely ballistic, ie dictated by gravity (if you like that sort of thing) or UA (if you don't). I'm going to go out on a limb and also assume that you also agree with with the reported flight time of 53 minutes, although do please let me know if not.
The reason the experts are coming up with ICBM ranges for the missile, despite it flying a relatively short distance, is that if something flies ballistically for that long and only travels 950km, it has to have a very steep launch angle. Indeed, the reported apogee was very high - 4500km. If it was launched at a shallower angle, it would go much further. But you don't need me to tell you that - as you keep saying, the equations are very simple. Even if you dispense with heretical things like the reduction in g as you get away from the earth, if you calculate the speed necessary at 'launch' (more accurately, the end of burn, but let's keep it simple) to fly ballistically for something like 50 minutes (the reported burn time was around 5 minutes), and then try a shallower launch angle, you'll get much bigger ranges than 950km.
If you are genuinely intellectually curious, then do the maths and let us know how you get on.