I think I started doing this around early October-ish, when that 'Dropchance of Premium characters' thread was starting up. From what I got, there seems to be a 4% (probably 5% but my luck stinks) chance of getting a 3* character with a Common Soul, and a 12,5% chance of a 1* character. I also tracked if there was a percentage attached to the class you get from a soul, and considering the chances of getting a Warrior, Sentinel or Magus is roughly 33%, I think there's a thing there which assures you don't get an overload of characters from the same class.
Also thanks to Blade685 as 99 of the souls in here are his. Helped me speed this up a bit.

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Hope you all find this interesting!

It'll be great to stop doing this though, it was such drudgework