Hi Obelix
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I don’t have a Loos gauge yet, I have ordered one from Amazon.com. I mentioned this in a separate recent thread on the topic. So I don’t yet have experience with it, but to be fair, I would be lying to you if I said I was unfamiliar with its application. Maybe some of those who have a gauge can chime in with some more applicable numbers that they have used. I am really looking more forward to acquiring the knowledge that will accompany having and using it, than I am to actually putting it to use tensioning my rigging. (It's my Christmas present, actually)
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My initial tensioning (beyond dealer settings, which were way too loose) was done by measuring the physical increase in distance between 2 pieces of tape (elongation a function of tension) on a segment of the top shrouds. The method is exact, but the Mac manual (as I recall) suggests tensioning on what would be considered the low side of what is average for the industry, so the values I chose to use were not so exact. Plus the tensioning has to be done in steps anyway because it's the nature of the incremental vernier tensioners at the chainplates on the Mac.
I have a (CDI) jib furler, so my forestay tension is not as easy to access and measure as the top shroud or the lower shrouds. The function of the forestay turnbuckle is really to set the mast rake angle, but if you wanted to be fussy (and I’m relatively not in regard to this) I would suppose you could make small adjustments in the forestay turnbuckle that would allow tension settings in the top shrouds in between what you would get with the vernier adjusters alone, and still maintain your mast rake within the desired limits.
But the tension on the forestay is determined by the design of the rigging to be a certain constant ratio relative to the top shrouds. Because of this, and to offset the mechanical disadvantage that aft-swept spreaders stipulate, and the lack of a backstay, the tension in the shrouds needs to be greater than with inline spreaders (like the X has).
In my boat I plan to use no less than the numbers you said, no lower than 200 lbs for the top shrouds, unless I get too much slack / curvature when under sail, then I will retension higher. This is starting at the lower end so you might want to go on the higher side of this if it works out practically.
Note that the generic norm for this (aft-swept spreaders) is 15 to 20%, or up to 25% if shrouds are sagging, according to a tuning book I reference. (Illustrated Sail and Rig Tuning by Ivar Dudekam, which I highly recommend- I use Amazon.com). As easy reading as it gets, there are others.
When you tension the top shrouds the spreaders will pull the mast forward in the middle, causing it to bend. The lower shrouds are tensioned to what they need to be to straighten it to bring the mast bend back to the desired amount.
The amount of mast bend you want depends on a lot of things (sail shape and trim, desired weather helm with rake, everything else we just discussed, etc). Pull the halyard straight, taut and flush with the mast at the top and bottom and you can get an idea of the amount of bend by the gap at the middle of the mast. Maximum never to exceed 1.5 X mast diameter.
Expect that you will be dancing back and forth a bit between all these adjustments, because they are all interrelated, and of course you will want to watch port to starboard symmetry at every point too.
Hope this answers your question. When I launch again in the spring (it’s in my driveway, then into storage in a few weeks) I will get some experience – so maybe some feedback from you after you have a go at it would be helpful. Thanks.
Regards- Brian.
rig for the first time, what actual tension on the LOOS Gage do you use for your upper and your lower shrouds 
