Method 2: The Titius-Bode Relation
This method is most effective for designing planetary orbits for Solar analog stars with luminosities ≤2.8 times that of the Sun; higher luminosity values begin to produce starting orbital distances beginning significantly beyond the Innermost Stable orbit; luminosities more than ~3.5 times that of the Sun begin producing starting orbital distances greater than the nucleal habitable zone orbit.
Below is a table showing the values for the Solar System as predicted by Titius-Bode, compared to the actual orbital distances for the planets:
Note: negative numbers in the difference column indicate where the Titius–Bode relation “overestimates” the actual Solar System planetary orbit.
The reason(s) for Neptune’s “break” with the Titius-Bode relation are still not completely explained to everyone’s satisfaction, though it is probably due to migrations of the giant planets in the outer Solar System. When Pluto  was discovered in 1930, its average orbit of 39.44 AU was in closer agreement with Titius-Bode's prediction of a planet at 38.80 AU, but that did nothing to explain Neptune. As a result, the Titius–Bode relation was dismissed for nearly 150 years as being a mathematical coincidence, but not having any real value to astronomers and planetary scientists.
However, Steffen Jacobsen  and others revived it in the early 1990s to see if it might be useful as a generalized model to aid in the search for exoplanets. As it turns out, Titius-Bode has been reasonably accurate  in predicting where planets would be found around other stars. Whether the effectiveness of generalized Titius-Bode relationships is due to resonances, or some other factor is yet to be clearly defined, however.
From Relation to Equation
However, using this equation to determine orbits for Worldbuilding means that the intervals and gaps between orbits will be the same for all the star systems you generate. Also, using this method in its basic form means that no orbit will ever be generated that is closer than 0.4 AU to the central mass of the system.
But, huge numbers of exosystems have revealed planets orbiting much closer to their host stars than Mercury orbits the Sun: Kepler 42c, for instance, has a semi-major axis of ~0.00585 AU , less than one-sixtieth of Mercury's orbital distance from the Sun.
For these reasons, I discuss below a couple of possible modifications to the Titius-Bode equation which make it more generally useful, and take into account relevant characteristics of the star when determining planetary orbits.
Alternative Equation 1
First, we can modify the equation to include the luminosity of the host star(s) as a multiplier:
- Starting at Planet 7 (ordinal 5), the orbital intervals all hover in the range [1.92, 2.00], which may seem a bit unnatural; however, this is a recognized feature of the Titius–Bode relation, a feature also mirrored in the Solar System.
- Even more unnatural seeming, almost all of the orbital gaps are exactly twice the magnitude of the preceding one; nevertheless, this is also a recognized feature of the Titius–Bode relation.
- The orbital distances become prohibitively large beyond Planet 7 (ordinal 5).
Alternative Equation 2
So, our modified equation would be:
This modification of the equation serves the need to fill in the gap between the first orbit and the Innermost Stable Orbit.
Planets 10, 11, and 12 (ordinals 8, 9, and 10) are much farther out than is really useful so, we can simply drop them from the list.
Let’s say we have a star called Anra, with a luminosity of 0.76 solar units. Because Anra's luminosity is <1.0 times that of the Sun, we use its mass to calculate the Innermost Stable Orbit, which then would be:
Any of three equations may be used, depending on the needs of the Worldbuilder:
3. 83% of the time; 124 out of 151 tested Kepler discovered exoplanet systems have planetary orbits closely in agreement with Titius-Bode predictions.