Here is a interesting article in Practical Sailor regarding tunning.
Practical Sailor - Boat Clinic: Tuning the Masthead Rig - Tips Article
It says: "If you have absolutely no slack in the leeward Cap Shrouds going upwind in 12 knots of wind, the shrouds are probably too tight for most boats."
I just tunned using the article and seemed to reach the prescribed tension (according to Loos), and I've got a lot of slop in the Leeward shrouds in those conditions. The PS article also differs for the technique used by Dedekam in several respects. I'll bet if you asked three different Riggers, you would often get three different opinions. So, paying someone to do it isn't necessarily the answer either!