I'm a bit confused.
The difficulties are listed as rising in steps of three (3, 6, 9, ...) while the degrees of success are listed as steps of four?
Wouldn't it be much more simple and intuitive to have a common stepping, so a Formidable (12) Test which you need two DoS'es on would equal a Very Hard (18 ) test? (using a stepping of three in this example)
As is, this test requires you to make a Test which reaches 20. Harder than "Very Hard" but not as hard as "Heroic".
If Tests can be set at every rating and not just the "named" ones, why bother with the names at all?