Migration is also a thing, remember - Planets form further out and spiral in. So either make it possible start off with a minimum orbit that is very small (e.g. 0.05 AU), or start with larger orbits and then apply a variable migration adjustment (e.g. reduce orbit size by 1-99%).
And it´s not just that, right? I seem to recall you saying that Jupiter, for example, migrated inwards, disrupting what is now the asteroid belt, then migrated back out, if perhaps not precisely to where it used to be. Messy, messy, messy...
[aprilfools]This is just too complicated for me. I think I´m going to give up on SF altogether and switch to heroic fantasy.[/aprilfools]
I think I´m starting to get a grip on how to handle this, though.
Instead of working with orbital radii in AU or million kilometers and black body temperatures expressed in Kelvin, I will use abstract orbits and BBT on a logarithmic scale - so that for every 10 orbits, orbital radius is doubled, and for every 5 orbits (and every 5 points of abstract BBT differential), energy received is halved. An orbit´s abstract BBT could then easily be calculated by taking the orbit 0 BBT and subtracting X (I´m leaning towards 1) per orbit out from orbit 0, since orbits and abstract BBT are both on the same scale.
Place the innermost planet on orbit 0, and the next planet 1d6+4 orbits out from the previous one, which on this scale corresponds to a minimum of 1.42-etc times the orbital radius and a maximum of 2 times the orbital radius. That (if we assume the asteroid belt to occupy an orbit somewhere between 2.6 and 2.8 AU radius) allows for the observed orbital radii in our solar system as possible random results - the relation of orbital radii between two neighboring planets is always somewhere between these two numbers.
Orbit 0, then, is either slightly outside the star´s Roche limit, or the point at which black body temperature allows for a solid planet to form. Probably the former for M-class dwarves and the latter for everything else. For close binaries and circumbinary orbits of near binaries, it is instead the closest stable orbit around that binary pair.