Bam, thanks for your input. nice boat.
We use spars with 50-75cm prebend laminated into them. By tensioning the forestay, the mast is straightened and effectively pulled in its shrouds for sailing. This is the only way to get some forestay tension going on a spreaderless 18m rig with a 75cm shroudbase. (runners are not allowed)
To get a good base set-up, we want to have a certain tension (1200dN) in the forestay (design sag of the jibs) with equal tension in the shrouds. that's why we need some sort of gauge.
Instead of trying to tune the mast for headstay sag using shroud tension forces, why not get a piece of small vectran or prestretched dyneema and pull that parallel, with lots of tension, to the headstay from the same attachment points; headstay sag can then be measured (ok, eyeballed) to suit the sails.
Also, isn't your shroud tension a dynamic changing number depending on wind pressure; wont that be affecting the headstay tension while you sail; the more wind, the more headstay sag - right? Are you adjusting the tension as you sail? How do you plan to take that into account?
While I am at it, Im not seeing 750mm of prebend, perhaps you meant 75mm. From what I can see, shrouds that all terminate at almost the same location, at or near the top of the spar. Can you explain how prebend is effected by the shrouds without having aft swept spreaders? Perhaps by the tension in the leech of the main, but without a backstay (which I dont see) changing the bend of the mast would be difficult, unless the mast was keel stepped, and you were bending the mast forward around a deck partner, something of which I havent seen before, except down wind in a star or an etchells.