In the typical left or right layback (not undercling), the general rule of thumb is if left-hand layback, then to prevent barndoor, 'plant' with right foot, mostly use mid-sole and outside edge of (right) shoe. If right-hand layback, then 'plant' with left foot, again mostly using mid-sole and outside edge of (left) shoe.
There are many refinements one can add to the basics. For example, one that works for me when the 'plant' foot (or right foot when left-hand layback) is on a marginal hold, is to brace the top of the toe box of my other foot/shoe (left in this example) against the wall. This creates counter pressure on the 'plant' foot to make the marginal footing (right in this example) a little better. But, it takes some practice to get it to work and YMMV too.
People often laugh when I say sometimes an 'inside flag' with the right foot works well to prevent barn-door when both left foot and left hand are used in a lay back move. Whatever .... but it works for me in certain situations and for Joe L too, from whom I learnt this unusual move almost twenty years ago.
The only memorable (read long) route with a lot of layback for me is Return To Forever at Acadia NP in ME. Good climbing, great location, and wonderful views from the top. Do it!!
