Shroud tension question

Ian,

In relation to your response, I have a question. How does one ensure the masthead is aligned directly above the centerline of the boat (beam to beam) and in the middle of the boat (stem to stern)? I have heard that you take a spare halyard or other line attached to the masthead and measure the distance from that to the stem, then compare that measurement to the same halyard led to the stern. However, in order for that to work, the attahment at the masthead would have to be precisely at the center of the masthead no? Side to side I can see it working, since the main halyard should come out of the sheave in the middle of the mast, therefore giving you a good comparison from one beam to the other. But how do you do it fore and aft? And how while under sail, crashing along on your ear?


Jim