I really love the paper selection it's a very Pretty book.. What I've learned from purchasing other people's work you need to let at least an inch or 2" on both sides of the hinge plate otherwise they will pull up because of the action of the Pages. Also your correct you should have accounted for a lot more Overhang for the spine.
Lovely and clear instructions on how to make the covers and spine for a good sized Mini Album, and I hope you won't mind that I shared your tutorial on my FB page Memory Book Crafting