You don't have to worry about not being able to carbonate after secondary unless it's a really long secondary. I've bottled cider after a 5 month secondary and even though it took a little longer to carbonate than normal, there were no problems.
I've heard that it can often take 2-3 pounds of fresh/canned fruit to impart noticeable flavour on a beer, so you might want to start with 1.5-2 pounds for a cider. That's just a guess though ha ha.
You don't have to worry about bottling at a certain gravity to make sure that it carbonates. You just need to make sure that it is done fermenting. Don't bottle it before it is finished or else you may end up with an unpleasantly over-carbonated cider that could break weaker bottles. Also, look into how much cane sugar to use in place of 5 ounces of dextrose. I know that cane sugar is slightly more fermentable than dextrose and that there is a thread on HBT, which has a conversion chart, 'er something ha ha ha.