IMHO there is no such thing as an optimum, location and design are all a set of trade-offs, what tradeoffs do you want to make?
The sheet length and effort involved in your mainsheet doesn't (or at least should not) change as the sheet ing position moves forward on the boom, you just add more turns to the tackle so the sheet-end load remains about the same. The extra turns eat up the sheet length a mid boom location would otherwise free up.
As to to the technical questions of % load and number of turns, I suggest you slough thru the thread
http://www.sailnet.com/forums/gear-m...insheet-2.html whcih really works the subject over.
Cameron, I gotta apologize for not paying attention, but I know that your boat already has a mid-cockpit traveler, why consider moving the traveler from the current location? Run out of boat chores?