I have never seen the exam, and suspect there was some sort of non-disclosure about public discussion of the exam problems -
but my guess would be a secondary auxiliary view was required.
If my guess is correct, this can be accomplished two ways, the hard way in the drawing, or the easy way using the Normal To in the part (you might save this as a named view for the drawing).
Did I guess close?
I believe this way you described is a viable option. It is not the way I think they want you to know how to do it in the test. However, for all intent and purpose, your suggestion allows one to gain the correct measurement. (Answer)