In relation to the S/D/T calculations, I am not sure how much importance there is on converting between Nautical Miles and Standard Miles.
As you would work in the unit of measurement given in the question itself. So if the distance is expressed as NM, the answer will be in NM and Vice Versa.
The obvious exception to the rule will be the ones asking for the conversion between the two measurements of distance.
Having said that, I haven't done stage one so am not completely up to speed with the types of questions asked. So if someone can correct me, that would be most appriciated, mainly so i can get practicing lol