You could travel quite a way with 12 days, but it sounds like you only want to travel about half those days and maybe not real hard on those days. How far might you travel in a day, considering that daylight is getting shorter by the end of Sept and that strong wind may shorten some travel days? Being flexible to travel when weather is good and sit tight when it deteriorates would be a good plan. You will quite likely have some cold nights (and days).
I like the suggestion by cyclones30, although you'll be on bigger water and wind may be a greater concern, especially if you must backtrack to your entry. You would generally be traveling west to east with prevailing winds, although they can obviously vary. The options for exit depend on whether shuttling is an option. If it is, a Mudro exit would possibly make a nice trip for you. If not, you'll have to backtrack to your entry point.
Voyageur Maps website has zoomable, scrollable maps that may help with planning. I sometimes use Fisher, McKenzie, or Voyageur maps, depending on the route. Sometimes you'll need 4 of 1 companies, but only 2 of another's. I also will often have maps from 2 different companies.