The leaders different widths because the rails are a different distance from where the fabric will be - the 24" is for the bottom rail - you'll attach the bottom of the backing to it.
I started with 3 leaders, and now I'm only using two - I load the quilt back (attach to the top and bottom leaders), load the batting then baste the entire top - baste along the top edge, smooth & roll and baste down the sides and then baste the bottom edge of the quilt top.
With everything basted, then your clamps on the sides are stretching all three layers together and evenly.