The spreaders on the mast affect how much and where the mast bends. Particularly the middle of the mast. It is important to have the spreader length and how much they are swept back 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 mast and make of sail what spreader measurements they use.
See WHERE DO I START for important information on pre-bend and matching it to the mainsail luff curve. The spreaders are critical in achieving this and will generally need different configuration for different mainsails.
If there is any play in the spreader bracket pull the spreaders back to where they be when shrouds are loaded to take the measurement.
Is measured from the shroud wire to the sidewall of the mast.
SPREADER POKE OR OFFSET
Is measured from the back of the mast track to a straight line between the spreader tips where the shroud wire is attached.
The Sailmaker or the Tuning Grids will be giving you the length and poke required to suit the luff curve built into the particular sail. It is worth measuring that for yourself to make sure it is correct. Any variation between boats may affect the Pre-bend measurement at the spreaders.
To see how much mast prebend there is look up a mast from the bottom or place the boat on its gunwale.