Questions regarding Specific Monk Style Feats

[TwitchyMcTwitch]

Snake Style.

A Combat/Style feat, reads as follows:

You watch your foe’s every movement and then punch through its defense.

Prerequisites: Improved Unarmed Strike, Acrobatics 1 rank, Sense Motive 3 ranks.

Benefit: You gain a +2 bonus on Sense Motive checks, and you can deal piercing damage with your unarmed strikes. While using the Snake Style feat, when an opponent targets you with a melee or ranged attack, you can spend an immediate action to make a Sense Motive check. You can use the result as your AC or touch AC against that attack. You must be aware of the attack and are not flat-footed.

Normal: An unarmed strike deals bludgeoning damage.

I have a few questions regarding the wording in this feat.

I have probably played too much Magic: The Gathering and may be over thinking the specific wording...

"when an opponent targets you..."

Does this mean the player must decide whether or not to use the Sense Motive check the moment the DM declares that the monster is going to Roll To-Hit? Or can the player wait until after the roll is made, but, before the results are announced?

Next,

"You can use the result as your AC or touch AC against that attack."

Does this statement provide/not provide an opt-out mechanism? In other words, can the player, having rolled Sense Motive; decide "nah, screw it, I rolled poorly, I want to keep my AC." ?

Finally, the above wording - does it work like a computer input, and replace your actual AC value for that single attack?

Specific reasoning behind such a pointed question -

Scenario:

Monk charges Bugbear, Monk is at +2 to hit and -2 AC. Two initiative ticks later, the Bugbear swings, the monk rolls Sense Motive and scores something slightly higher than his normal AC. Now, I adjudicate that his 'AC' is at -2 because of Charge action, thus, in my mind - since the Sense Motive check BECAME his AC; it should be at -2 to that Sense Motive roll.

Which is correct? (The opposite of my statement being that the Sense Motive check functions in PLACE of his AC, and thus is not effected by the charge.)

[Bardic Dave]

My gut reactions:

1: You have to declare before you see the die roll.
2: The ability does provide an opt-out. You can choose to use the sense motive roll in lieu of AC, but you don't have to.
3: To me, it makes sense for the same logic that applies to the bard's versatile performance to apply to Snake Style. The Devs are on record as saying circumstance penalties modify the final result in the case of versatile performance.The AC penalty from a charge seems to me to be like a circumstance penalty (although it isn't explicitly called out as such in the rules). Ergo, the -2 penalty would apply to your sense motive roll.

I'm curious to see how other people answer!