@Avionic Let me first say, that I don’t have the Starship (yet), so I’m merely spitballing here. It makes sense that you have to use a reference point not part of the flight plan, when the alternatives are point abeam and radial. That obviously can become a problem, if there are no appropriate points to reference.
In your specific case, as you say, a solution could’ve been, to work out, at what radial toward MIKSI did your flight path intersect, when the distance to TNO was exactly 20 Nm, or whatever distance you’d prefer. It would require some extra preparation, for sure, but I think it could be a fun exercise nonetheless. Love these questions, where you have to use a little bit of creativity.
Cheers, and good luck!