It is easy to think of standing rigging and running rigging as two different parts on a sailboat, but in fact, they both share the same goal: Allow your yacht to sail.
Running rigging is thought of more often with sailing as sailors use them to set, trim and adjust the sails. The standing rigging, on the other hand is left standing there.
The truth is, your standing rigging is only there to hold the mast in the proper position to achieve better sail shape. If the mast is too far forwards or backwards, it will lead to poor handling and sailing performance will suffer.
Back to the original question about tightness of stays. The answer is very simple, the stays need to be tight enough to hold the mast in the proper position. I know this sounds vague and ridiculous, but bare with me for a moment.
If your stays are at the proper tension, then the mast will remain in its ideal position on all points of sail.
The reason that the correct tension is not a number is because no two stays are the same. If the boat builder placed the chainplates at slightly different spots, then the tensions in each stay will differ to maintain the mast in the proper position.
Now that we have moved your mind away from a set number, lets look at the actual tightness of the stays.
If you don't want the mast to move, then all the stays would need to be bar tight! This would hold your mast still, but it would also stress the heck out of all your fittings and fasteners. Your mast might stay perfect for a few sails, but before long you will be cracking mast tangs and ripping chainplates through your deck!
You actually want your rigging to be a bit on the looser side of life, in fact, the ideal would be to have your standing rigging as slack as possible while still able to hold the mast in position.
Higher aspect ratio rigs will require higher static tensions to keep the mast in place as compared to lower aspect ratio rigs. This is easy to remember because you can switch out "aspect ratio" with "strung" and just imagine that the people on fancy race yachts are high strung while those on low aspect ratio cruisers are not high strung.
On a massive triple spreader race yacht, the shrouds will be tight as banjo strings to keep the mast from deflecting too much as the wind builds. On a gaff rig yacht with a short mast and no spreaders, the stays might wobble around a bit as they are rather loose without the sails set.
Once sailing, the force of the wind on the sails will pull the mast over and the stays will all tighten on the windward side and loosen on the leeward side. It is common to see, even on high aspect ratio rigs, that the leeward shrouds will dangle around loosely while sailing in a blow. This doesn't mean that the stays are too loose, simply that the mast is deflecting a bit to leeward, and that is just fine.
While you might want your mast to remain perfectly centered as you sail, which would make you want to tighten your rigging until something breaks, your sail maker doesn't want you to tighten them that much. The sailmaker is planning on your mast deflecting a bit, and builds your sails expecting this to occur. If you rigging is too tight, your sails will remain too flat and will not provide you with sufficient power; thus leading to poor sail performance.
Instead of thinking of your standing rigging as a static line that is set once and forgotten, it is better to think of your standing rigging as another line on your yacht that should be adjusted to improve sail shape.
To answer the original question of "How tight should the rigging be?" The cap shrouds will be the tightest, the intermediates (if present) will be a bit looser, the forward lowers will be a bit lower again, and the aft lowers will be the loosest. The tension of the cap shrouds is set to be "tight enough" that the spreaders don't rattle, and the mast doesn't pull hard to the side when you yank on the cap shrouds. A good starting point is where you push on it with a good yank and the stay only moves about half an inch.
Once you have this stay set, you then can feel its tightness and go setting all the other stays along the way. As you set the tension on the stays, you will want to sight the mast repeatedly to make sure the mast remains in column and centered over the yacht.
You also want to be certain to start at the top and work your way down the mast. This will make dock tuning the mast a one time thing, as you will set it roughly perfect on your first attempt.
Once you have the shrouds set, it is time to set the headstay and backstay. These stays share the same load of keeping the mast in position, fore-aft. The headstay is best set first, positioning the masthead slightly aft of the mast base as it meets the deck. This will make your mast rake aft just a bit. A good starting point is to look up your yachts design plans and see where the designer wants the mast head to be. If plans are not available, a good rule of thumb is to set the masthead aft the mast base 12 inches for every 50 feet. How to find this position will be discussed in a future post.
The tension of the headstay will be determined by the backstay, which pulls on the headstay via the mast head. Ideally, you want to have a backstay adjuster present that way you can adjust your headstay tension while you sail along.
Headstay tension is the last stay that should be thought of as a static setting. Windward performance depends on headstay tension. The tighter the headstay (to a point) the better your ability to point will be. The looser the headstay (to a point) the more power your headsail will be able to produce. As you bear off the wind, a looser headstay will power up your headsail and provide you more lee helm which will help to pull your boat downwind with speed.
Completing the dockside rigging tune is only the beginning of the rig tune, once this is completed, the next step will be the sail tuning. Sail tuning involves taking the yacht out for a test sail where you will tune the rigging as you sail to maximize your sail shape and performance.
At the end of your dockside tuning, you will never need to know a "number value" for the tension in your stays, instead, you will position the mast to a rough ideal and set the stays to a "good enough" setting that will allow you to safely go out sailing and do the fine tuning of your rigging.