Justifying TAG?

This really isn't a post; it's a question for any of our presuppers out there.

In the comment section of Acharya's introductory post, the topic turned to the transcendental argument for the existence of God (TAG).

Paul pointed out that TA's were justified according to modus ponens, so that

<> P-->Q
<> P
I'm assuming that P = universal laws of logic and Q = the Christian God, so that "If universal laws of logic exist, then the Christian God exists. Universal laws of logic exist, therefore the Christian God exists."

Normally, after making an argument, people seek to support each of their premises.

Now, correct me if I'm wrong (and, admittedly, I may be), but the way presuppositionalists normally attempt to justify the first premise (i.e. that the existence of universal laws of logic presupposes the existence of God) seems to be to say something along the line of "Prove to me that universal laws of logic can exist without God; you can't, therefore premise one is true."

This seems to me an odd way to "justify" a premise, i.e. by asking someone else to prove that it is wrong. That isn't to say that it is not a valid means for justifying, it just seems to be shirking the responsibility of justifying an argument you put forward.

Like I said, I may be wrong about how presuppers justify that premise. It seems, though, that this is what Bahnsen meant by his "impossibility of the contrary" arguments. Instead of saying, "In various forms, the fundamental argument advanced by the Christian apologist is that the Christian worldview is true because of the impossibility of the contrary," [see here] he should have said, "The fundamental argument advanced by the Christian apologist is that the first premise of TAG is true because of the impossibility of the contrary and, therefore, the Christian God exists."

I've argued with Manata before that it seems like Bahnsen's argument is more like:

P v Q

[I.e. "Non-Christian world view or Christian world view; not non-Christian world view, therefore, Christian world view."]

I still can't read the article I linked before without thinking that, but it seems to me that if the "impossibility of the contrary" argument is not at this stage of the debate, then it is definitely at play in the justification of the first premise of Manata's modus ponens argument.

Although I'm still not quite clear on how it would be particularly worded.

Help a brotha out, won't you? Tell me how you justify the first premise of TAG.

[As an aside, in another post, I attempted to demonstrate that the laws of logic are not necessarily universal. I have yet to see a detailed critique of why this cannot be the case.]