Thanks all for the replies so far. Special thanks to Knut for the drawings and the link to the Lewmar PDF.
So it sounds like the consensus is that nobody cares much about low stretch on a mainsheet. I can get on board with that and it certainly simplifies things. Glad to hear also that 1/2" seems to be a good candidate, that also makes things simpler and cheaper.
To the comment about "getting real" because 40mm blocks would induce too much friction, I find that this is exactly the kind of information that is hard to find in the product guides. So is a 40mm block too much friction because of something on MY boat, or is it too much friction for 1/2" line, or just too much friction for everyone? Why, if the block is rated for 1/2" line would they make a 40mm block at all? How will I know if the sheves are big enough to have a low enough friction level? If my choice is worthy of ridicule, is there a more salient guideline that I can be pointed to?
As many, and the Lewmar PDF have outlined, I'm missing the DECREASE in load that goes along with purchase. Yes, my brain knows that 6:1 means that you end up pulling 1/6 of the load, but somehow I didn't connect that it also means you can size some of the blocks for the reduced load they will see. Currently the mainsheet isn't on a winch at all, but my 6:1 purchase answers the question as to why my plastic cam-cleat hasn't exploded yet.
My new setup will be going from the boom, through a fiddle block a couple times, then forward to the mast, down to the deck, around a deck organizer, then to a rope clutch and finally a winch. Any thoughts on how much purchase I should have for my 400sqft sail before the winch?
This is what I have in mind, though I'm really hoping that I don't need a deck organizer near the mast AND at the winch....
Thanks for all the help so far.