Be sure to order F26 Adhesive (10.3 oz. per tube), specially formulated for use with Masonry Products.  It is an excellent, quick setting adhesive, specifically designed for use with masonry and natural stone and won't 'bleed through.  Size 1 (72" Hearths) sets typically need 3-4 tubes.  Size 2 (66" Hearths) sets typically need 3 tubes.   Just use a bead 1" or so from the outside edges, and a large 'x''  1" from each corner.

Also, look at our special CV230 Micore 300 Board, a dense, fibrous material to put under your stone hearth.