Okay, thanks for everyones patience.
I went out and measured my stock Giant post. I measured the back of the seatpost tube to the back of the actual seatpost head. I came up with 25 millimeters. Does that make sense? If I buy a 22 mm (setback) post will that work?

The length of the post is 330 millimeters. I can't even see the numbered adjustment marks on the post where I have it set at for my height. Should I go with a 250 mm post or 300 mm post?

One of the websites advised that the more seatpost inside the frame the more stable and strong the post.

I've read alot of good things about the Thompson posts but I wanted carbon for a smoother ride.