Best way to see it is for the solo ship to be the "compass rose". Everyone has their own way of visualising spatial oriented things but this works easiest for me. Keep in mind that the more precise you set the software the more accurate the manoeuvre will be. If you want to just "wing it" and have a quick guess at which rel. HDG you set then this is also fine. Not every human player puts a lot of thought in it.
F.Y.I. This rel. HDG setting helps the software extrapolate the target's position. So if you select 90-135 or 135-180 you will likely get similar results.