The 'practical/technical' sailing answer is: the tension should be that whatever required to keep the curved leading edge luff shape of the sail to the designed curve (called luff hollow) that the sailmaker cut into the sail.
A sailmaker will typically cut headsails expecting
that the wire is typically at approx. 15% ultimate strength tension for sailing in winds at approx. 15kts.; this 15% tension yields a fairly predictable wire 'sag' at those conditions. If the 'sag' in the wire doesnt approximately 'match' the 'hollow' in the leading edge ... you will have worse than piss poor pointing ability and upwind sailing characteristics; downwind well below a beam reach doesnt matter.
If the wind increases or decreases, a 'tech' sailor will change the headstay tension (via changing backstay, etc. tension) up or down so that the curve in the leading edge is restored back to 'as designed' shape ... or the boat will point like a pig, heel over aggressively, and can start aggressively skidding off to leeward.
Adding anyother additional stay in front of the mast will radically complicate matters --- "dynamic load sharing" of the 2 stays, where the stay that has a sail flying will unload into the stay that doesnt have a sail flying ... and the flying sail will be on a reduced tensioned and 'over-sagged off to leeward' stay.
How do I tension? If beating with the 'forward sail' flying, I manually unload
tension in the 'inner' stay by whatever it takes and amount until I SEE that the sail flying on the headstay has the correct forward shape in those specific wind/waves/amount of heel conditions. I then adjust backstay tension and/or apply running backstay tension .... all dependent of the shape of the sail and amount of 'sag' in the wire to match that leading edge shape of the sail. Since the conditions are non-constant and 'dynamic', there isnt any 'tension number' to give you.
For a solent rig, the above is ass backwards as normally you want the inner stay to be the pointing sail on the tightest stay and the 'outer' (more loose) stay to carry the downwind sail ... all ass backwards due to the structural geometries, rigging 'elasticity', etc. I wouldnt have a solent rigged boat for that reason, I sail a cutter rig and I can easily get that 'headstay' as tight as needed.
Only when sitting at a dock with the sails furled !!!!!! Otherwise in dynamic conditions those two forwards stays will be 'dynamically variable' and depending on the amount of sq. ft. of sail being flown. If both stays are sharing the original 2,500# load ... then BOTH sails will have to be recut for 15% / 2 average luff wire tension = 7.5% !!!!! ... and expect the mast top to be 'wandering' and 'swaying'.
Some distance sailors will determine (measure) the average tension in average conditions in the flown upwind sail's stay ... and then simply have a new and 'deeper' luff curve cut into the sail to match the 'average' resultant tension.
Strength of materials (service life) consideration: If you double the amount of wire in front of the mast, and dont have a means to structurally double the reaction loads by backstay (now having to operate at TWICE its design load), etc., you risk accelerated 'fatigue failure' of all wire 'aft' of the mast ... all due to that 'extra' forward stay, unless one has a safe means to UNLOAD one of the forward stays - IMO.
Hint: dont expect a single OEM design backstay to last very long when adding a 'solent stay' when youre applying high backstay loads to keep 'good' headsail shape. Stainless steel rigging has a fatigue endurance limit of only ~30,000 psi or about 30% tension, and if the loads are above this approx. value, you only get ~1 million 'load cycles' before the rig 'embrittles' and fails from 'fatigue'; .... keep the loads 'under' 30% and you get a very looooong service life out of stainless.
Static rigging loads ... only a 'starting point' for 'how much tension'. One has to remember that the forestay 'sag' (tension) is what establishes the all important SHAPE of the 'headsails' in varying wind and seastate conditions.
A better explanation of 'matching forestay sag to the leading edge curve': http://www.ftp.tognews.com/GoogleFil...f%20Hollow.pdf
Lastly, listen to that grumpy Bob Perry fellow; he DOES know what he's talking about ... I have one of his boats and when I back-calculate his work, I usually state out loud - 'holy ****, thats amazing'! :-)