The spreaders on the mast affect how much and when the mast bends. Particularly the middle of the mast. It is important to have the spreader length and how much they are swept back roughly correct to suit the luff round that is cut into your sail. Most mainstream sailmakers can provide you with the recommended measurements. See the Tuning Grids provided for details. Or ask one of the top sailors using the same make of sail what spreader measurements they use.
Is measured is measured from the shroud wire to the sidewall of the mast
Spreader Poke or Offset.
Is measured perpendicularly from the back of the mast track to a straight line between the spreader tips (where the shroud wire is fixed).
If there is play in the spreader bracket pull the spreaders back to where they will be when the shrouds are loaded during sailing.