Tax and Obstacle Modifiers

So according to the book the obstacle for tax on casting a spell is the spell’s obstacle. Does that mean base obstacle or modified obstacle? For instance, if the evil wizard is casting shards while the thief has positioning advantage with a knife he is at +3 Ob to his spell. Is he also at +3 Ob to his tax roll after?

I tried using the search function to see if this has been answered but apparently it doesn’t like the word “tax”.

Spell obstacle, not casting obstacle.

And any penalty from the casting hastily rules. And I do believe armor penalties may apply, right? Because tax counts as “fatigue?”