A: Spring angles are 45/45 deg. For a right triangle with a 4-5/8 hypotenuse, each leg computes to 3.27 inches. Call it 3-1/4! On my moulding jig, I needed to set the guide to a "tight" 3-1/4 in. Each specific piece dimension varies.
A: Rich, To better answer your question, please contact our HD.com customer service department at 800-430-3376 call any day between 6 a.m. and 2 a.m. EST, or call your local store, they can either answer your question or transfer you to your local store to answer your specific question.
A: glue a furring strip into the corner leaving about 6" on each end open, then just nail it to the furring strip. 1x2 strips work best. By now, you're done I hope!
A: Sorry, I should qualify my answer...when I say glue a furring strip into the corner, I mean into the corner where the wall meets the ceiling along the length where you're installing the crown. Leaving a 6" space at the corners where the walls meet will allow you to adjust the joint of the adjoining crowns.
A: Use 18 gauge at least 1 1/2 long. Shoot nails on as much angle as you can. Alternating angle with each shot. This should hold in place pretty secure. Once caulked it's done.