I’ve made a couple of large book and notebook covers, so if you want to chat more feel free to message me. You’ll be looking at quite a few stitches, a standard 3-4 mm diamond setup would be perfect. Honestly for the sizes of the standard binder I don’t think you’ll be able to use the proof grade leather since it’s not big enough. There are a few different size recommendations I have that might help in regards to the thickness of the leather, but in the end you’ll want what what works your budget best. There are some snafu’s I’ve ran into on my last project that I’d recommend you check out my post on it here
Now that solution might not be the best for your situation and that’s fine but there is much I learned from this petticuler instance that really did help shape my projects going forward.