32–36 inches from the floor (or 1/3 of wall height). Lower (24–30 inches) for a more traditional wainscoting look.

Caulk: Apply paintable latex caulk and smooth with a damp finger. Shims: Use thin wood shims behind loose moulding before nailing.

Yes, but: Sand down high spots or use construction adhesive for extra hold. Consider backer rod behind moulding to fill uneven gaps before caulking.