Hello Big Mac
Some of the earlier ranks appear to have an arbitrary post range (or at least I can't seem to find a correlation between them).
The other main numerical pattern I've spotted is between the ranks of 'Troll' and 'Cloud Giant', where the posting range is n + 50, but the range is only increased on every other rank. So for 'Troll' and 'Hill Giant', the posting range is 100 + 50 = 150, and for the next two ranks of Stone Giant and Frost Giant, it is 150 + 50 = 200. Then it is 200 + 50 = 250 for Fire Giant and Cloud Giant. Note that for Storm Giant, it switches to n + 250 (in this case 250 + 250 =500), which is what the posting range formula is for the dragon ranks.
Looking at the above, it appears that the highest rank of each category switches to the posting range of the next category. For example, the rank of troll (the most powerful creature before giant) follows the giant rankings, whilst the storm giant rank (the most powerful giant) follows the dragon posting range. This therefore suggests that the Gold Dragon rank might not actually follow the dragon posting range at all, as the Gold Dragon is the most powerful dragon in the Basic D&D rule set. Therefore, it will most likely follow the posting range for the next highest category, so will begin at 8,250 as I stated previously, but will probably not finish at 10,250 (and instead will be a higher number).
As for the post ranking group above 'Dragon', I suspect that they will follow the Immortal rule set rankings of Initiate, Temporal, Celestial, Empyreal, Eternal and Hierarch. This however is complete conjecture on my part!