The other side of that spectrum is when your fingers just type, without you thinking at all about it at all. I'm sure you have that experience with a standard keyboard. You can get to that point with the Twiddler too.
The answer to that question is pretty complicated. I spent weeks writing and running search algorithms looking for a set of current MCCs. It's been almost 2 years since I did it though, so I'm a little rusty on details.
The short version is the current list of MCCs in Backspice is optimized for frequency and overlap (I used to refer to overlaps as "confusions").
The rules of typing used to be hard, but in Backspice2 they are very simple.
There's actually only one rule: Always type the first MCC that you encounter.
So type h-is-t-or-y, ot-h-er, b-es-t, etc.
Actually there are not that many.
This is the whole list (I'm not sure it's completely accurate though):
ith=3440, est=3080, ist=2045, ast=1224, oth=1175, ach=836, eth=573, ath=466, sth=11
As soon as you add one MCC like "to" or "re" (the one you have mentioned), the number of confusions is going to increase drastically.
Thus those frequent MCCs have been removed from the layout.
8000 (aka 8k) is about 0.28%.
8000 was a round, well-chosen number that I used in my search algorithms.