My impression of the set up on the 20 is that you only need a little prebend when the mast is bare. Its the down-haul that is used to control the mast bend.
It is vital that the diamond wires are really tight to stop the mast bending sideways even the smallest amount, this would close the jib slot and undo the whole reason why the prebent rig was developed.
So my thinking was to set the spreaders to the minimum rake 1.5" with high diamond tension [can't remember the #s], Then if you can't get max bend with the down-haul [to match the luff curve] you need to start again with more spreader rake.
What I am trying to say is that the spreader setting is the last one to be dialed in.