How do I know which setback to choose from?

A setback allows for the fulcrum point to change so that a motor can lift the hull quickly without straining too much while keeping the hull on plane with relative ease.  Each hull is different due to hull variations, weight carried, motor weight, rigging weight etc.  When adding a plate, please consider that a motor will be set back further, pushing your stern lower in the water.  Consider comparing your boat to similar boats with jackplates installed to select the appropriate setback.