So to add to the confusion...when setting spreader rake, we typically measure by placing a straight edge (ie batten) across the two diamond cables at the spreader tips, then measure the distance from the straight edge to the mast track. When we're at the rake distance we want, then we crank on diamond tension (using a Loos guage). Typically, if we re-measure the rake setting as above, it does not change after diamond tensioning. So, to check pre-bend, you're suppose to stretch a line from mast top to mast foot and measure from the line to the mast track at the spreaders. This should change with differing tensions (the ends of the mast are flexing in the fore/aft direction with less/more tension, respectively).

I have never done the pre-bend measurement...since it can't be measured easily without tipping the boat on it's side and I'm too lazy :-)