Rig shouldnt be sagging that much in 10knots. wind up your caps say 8 turns a side, forestay about 12-15. do this with your main up and the boat on the trailer, once you get the mast in column with your luff round and you have the rake correct, wind up your lowers by hand, then plus 2 turns. check the mast is straight across the boat. i can't tell if your top stays just go from the masthead, through the spreaders and straight to the chainplates of if they are discontinuous, but if they are the first mentioned then wind them on till they are close to hand tight and back them off 2turns. no real reason to put in an adjustable forestay because you can't get rig tension. you should be able to get plenty of rig tension without one, otherwise your boat is bending like a demon which i heavily doubt it is. caps and forestay = rig tension and rake. lowers keep the mast in column and adjust main depth in light and heavy breeze. you can pull your forestay on till the cows come home, and yes it will tighten the rig but the centre of effort will more forward etc. just wond it on before you do a big change like that.
Sorry, not a native Eng speaker - what are "caps"?