There are really no hard and fast rules of strictly "BY THE NUMBERS" for rigging, other than
1. the rig provides a stable platform for the 'shaping' of the sails
2. The mast remains in a relative 'straight column' ... as a platform for the sails. (Although, there are rigs that are purposely built as 'bendy'.)
2. the stretch (sag) of the rigging wire (forestay, etc.) provides a predictable structure to which one attaches the headsail. Sail design, especially the wind-loaded headsail, is cut/designed based on the predictable stretch/sag of this wire.
What this means is that typical rigging, may have material strength/stretch characteristics that falls on one extreme end of 'the specification' especially and importantly it's 'elasticity'; and, to set up for optimum/maximum performance of the sails (due to their spread of tolerances/dimensions due to variability of the cutting/design and variability in the sail fabric material strength characteristics, etc.), that the rig and its set tension may have to be varied so that the sail characteristics MATCH the structural/elastic rig characteristics for the range of windstrengths as based on the design. TWO
independent 'systems' that must be adjusted so that they 'work' in concert
with each other.
All 'the numbers' are based on averages - average stretch of the wire, average stretch of the sail fabric ... when wind-loaded; and, sometimes the way that all these 'averages' stack-up, one may have to operate such tensions at outside of the 'averages'. Unless specified otherwise, such as for high end racing, etc., the typical sail will be designed for use between 12-18kts of windstrength. On either side of this 'normal' (15% of wire tension or less than 10kts. or greater than 15-18kts) the precise sailor will have to make adjustments in rig tension (typically backstay), sail (edge - luff/foot/leech) tension, etc. etc. etc. etc. The precise sailor wishing for 'maximum' output and a high level of performance usually will make such 'adjustments', sometime beyond 'normal' to achieve this goal. If you're not going to set up these tensions yourself, find a rigger who is also a sailmaker or vice versa!!!!!!!!
An Example: The 'usual' situation where rig tension (usually backstay, etc.) should be adjusted is when operating at or above the maximum 'design' condition parameters - beating in 18+ kts of wind and where the rig stretches in dimensions ..... and now a large sagging curve develops at the luff of the headsail (a big curve in the luff to rearward and to leewards) .... such changes the angle of attack which decreases ability to 'point', the leewards sag promotes aggressive heeling, slows the boat due to increased sail draft and hooked up leech, and promotes the boat to be now start skidding off to leewards because the sail went 'draft aft'. The solution: change to smaller jib, or tighten up the backstay, change the 'edge' tension on the sail, less sheet tension, etc. etc. BUT not to increase the tension so that the rig exceeds 'yield' (typically above 30% tension in 'some' member) or the rig will be in danger of breaking.
If you want to continually be at the top of your racing game, record all the tensions vs. windstrengths, etc. ... especially when you 'won' and when you 'lost' - do not 'guess'.
Here's an article that I wrote many years ago which discusses the phenomenon of sail shape vs. rig stretch (rig tension). This article was written for cutter/solent rigs which have 'very complex' stretch characteristic due to the 'interplay' of two headsails; but, the principals will apply to simple 'sloops'.
Rx: set your rig tension so that the shape of the sails, especially the luff curve of your headsails, closely matches what the sailmaker designed into the sails, AND that the rig tension does not routinely exceed 30% when the boat is at maximum heel when in 'high' wind ranges. (in 'blammo/survival' conditions when on long passages, my tension meter is mounted ON the backstay - tells me WHEN to reef down or change to a smaller headsail, etc.): http://www.ftp.tognews.com/GoogleFil...f%20Hollow.pdf
15% is the 'usual' starting point for rigging ... for Sailmakers too
Hope this helps.