Also measure side to side and find out if the mast is in the middle of the boat. Using a Loose Tension Gauge, tighten the upper shrouds to 20 and the lowers to 15. Adjust the backstay bridle turnbuckles so that the roller is 10 below the connector plate. This will be the base setting. You will need to move forward or aft until you get the desired pre bend.
Pre bend is measured holding the main halyard directly to the gooseneck.
Measure at spreader height. In heavy air, bend can be achieved through the backstay tension. Backstay tension will bend the upper part of the mast and increase headstay tension, flattening the genoa. Once the wind lightens you will ease shroud tension, this will increase headstay sag, improving pointing ability and gaining power. As the wind picks up progressively, you will tighten the lowers more than the uppers.