OK, this might not be exactly the answer you were looking for, but it was the easiest way I could think to explain it. I've made 4 shared ink books now...
If I make a 12x12 book, I format my pages to be 12x12, understanding that a little of the edges is going to get cut off. If it's a full bleed, I just make the picture go off the page a little. If it's a bordered page, I make sure I have at least 1/2 inch all the way around, an inch is preferable.
Does that help at all? I'd have to dig a little to find out if the measurements you're talking about are correct, and I think the way I do it is a little easier...
Just make sure you email Chris and ask for your full-bleed pages if you don't have a pro account.