What is the proper placement of the hull of a boat on the support planks of a boat lift?

This is more an art than science. The rule of thumb, is that the support planks should be equal distant from the center line of the keel, and about 1/3 of the max. beam of the boat at it's widest point. Then the boat should be far enough forward that the transom is about a foot or so from the aft end of the plank, but making maximum contact with the hull. It's that last part that causes the problems, That's why some lifts have multiple support planks set at various widths.