For example. the spell "wither" asks you to take lore test then flip the card.

On the reverse it says

"Resolve the effect based on your test result:

0:

1:

2+:

"

What number am I using here? The total from the dice rolled for the lore test? So, for example, could I fail the lore test by rolling a single 3 but still gain the results of 2+ from the reverse?

The total number of SUCCESSES (5 or 6) from the Lore test are used to ref the flip side and if you don't get any, you don't gain the initial +3 Str bonus on the front side, as well as suffer the effects on the flip side.

Compare your successes against the listed effects ... simple as?

The text can be a bit confusing sometimes, and there has been odd "mishaps" from how they've written odd cards, but this one's straight forward if you follow each card side in turn,

Walts

Makes sense, thanks!

So if you fail, resolve for the 0 result.

Not that if you succeed on the roll, the effect goes off, even if the effect for 1+ is the same as for 0.