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Rig tuning Mirage 24
The lines you mention (shrouds, forestay, backstay) which hold up the mast are actually referred to as the "standing" rigging; the running rigging consists of sheets, halyards, etc. which control the sails.
Since you posted this under the "Racing" heading, are you looking for some tips on improving speed? Learning how to use mast bend and forestay/backstay tension control to optimize your performance can be very helpful whether you are a cruiser or a racer.
You should know that there is no particular reason (other than caution) why the standing rigging on a boat need be tightened substantially less than its maximum breaking strength; racing boats such as the America''s cup boats(according to comments that came with a borrowed LOOS tension gauge) work with very low safety factors (1.5, say), so they tighten their shrouds and stays fairly close to the breaking strength. But you probably won''t want to push the envelope that much, particularly if you have an older boat with stainless wire and swaged terminal fittings. You don''t always get a warning when these are about to fail.
Tension of 15% of breaking strength of your main shrouds and stays (and about 10% for the intermediate fore and aft shrouds which end halfway up the mast) is probably a good "rule of thumb" value to start with, anyway.
Remember to sight the mast to make sure it''s not bent, and also use a halyard to measure whether it is centered side to side (mark where the halyard touches the rail on port side of deck, then see if it touches the starboard rail at the same spot). You can also do "dynamic" tuning, i.e. check to see if there is excessive slack in the shrouds when sailing to windward in a good breeze.
If you are tuning after stepping the mast, hopefully you can replicate the length of the forestay as it used to be so that your rig is "balanced", i.e. mast rake is close to designed value. If it is not, you will probably notice greater or less weather helm (or lee helm) and can ajust the forestay and backstay accordingly to get things back in balance.