Well, I can't comment on your math, but, I'm also currently holding the Cdn tix, and my TA has advised, that if FD is a better deal, we have to buy the full price US tix with PH......
We're a family of 5 (2A, 3C), and so far, as long as a comparable fall RO discount materializes, FD won't be our cheapest option (because we have to give up our discounted Cdn tix, and buy PH, which we wouldn't normally buy). With 6 Disney adults....FD is probably the better deal. Are you getting more than one room at BLT? I saw other posters mention about doing two separate packages, one with a RO discount, and one with the deluxe dining plan (then sharing meals)