You should be able to "strum" a note on it (opinions diverge on whether it's an F or a G, you could compromise on F#, I guess
).
That amount of tension will leave the lee shroud visibly slack going upwind in 10 knots or more.
If you rig a jib halyard tensioner, you can fine-tune the equivalent of the forestay tension - your jib should have a wire luff that will take over the tension from the actual forestay when the halyard is tensioned.
Adding that tension would add a little bit of tension (in reaction) to the shrouds, but most of their tension is in opposing each other.
What I do, is to set things up to the point I described above, and when conditions promise to be a bit breezier, I'll add another turn or two on my mast jack (pretty coarse threads on that one, so probably equivalent to 3-4 turns on the turnbuckles). Won't claim to be an expert, but that feels about right to me.
At one point, after getting my replacement rigging, I had a friend bring a Loos gauge and we tensioned to a value from some tuning guide. That required 8 turns on my jack, I usually to that far only when setting up for stronger winds, that is, most of the time, I back off by about 2 turns. I've concluded that this means that my "feels right" and the tuning guide are in the same ball-park, which is good enough for me.
Would be interested if anyone out there has experimented with different methods or has some definite preferred tension values (or perhaps knows a tuning guide that does - I lost the link to the one I used).