Whopity is spot on in that the thumb rule (max drift is half the crosswind, or whatever) is good enough for PPL level flying (dead reckoning).
This is because a) most people fly with non-slaved compass (DI) systems whose gyros are shagged and b) the forecast winds aloft are usually way off.
But one has to pass the exams somehow, and the options are rigged to assume the use of the slide rule and in particular the second (iterative) step. I think there is an alternative way to use the slide rule which avoids the iterative step(s) but it isn't normally taught and would probably fail you the exam
Similarly if you do the trig with a calculator (which is pretty trivial) you might fail the exam if they give you two answers 2 degrees apart and your calculation lands between the two