Seems like you are asking about 2 different bends;
Prebend is the amount of spreader rake/diamond wire tension that is induced in the mast in a fore/aft plane without the sail even being hoisted.

Max mast bend is induced beyond that by mainsheet and dowm haul tension on the sail. Take the battens out of your mainsail[or loosen them] and lay it on a flat floor and look at the curve of the luff - this is the max bend that you are trying to get to.
i know that the latest 5.7s are being rigged with prebend [ie very tight diamonds] but I'm sure that yours was not designed with this in mind. Someone will need to chip in here to let us know if it works on the older rig.

Paul