I was PhD in Solid state physics two lives ago :)
Now i almost forgot how to divide with piece of paper and pen, but 727 gives me some chance to recall it, coz i dont use calculator inflight :)

As for real world, - NDB approaches not precise, so this numbers are used to have at least some control over approach and to stay within safe minimums, and surely in bad weather it helps not to get lost :)

So just quick calc will do the job fine :)

P.S. Lately i find out that VFR flying require much more planning using Google Sat photos  :o
I did VFR YBLN to YCUN in DC3, took a wrong turn on highway, and got lost :) No triangles could help in that case  ;D