Estimating the amount of wool stuffing needed for a project
It seems, from casting about online, that the use of wool stuffing for refurbishing furniture cushions is somewhat outmoded, even for a restoration project such as yours where authenticity is desired. Other than several blogs mentioning wool when discussing the pros and cons of various cushion materials, especially those which are emphasizing natural furniture materials for health and green reasons, there is not much expertise being shared online on this topic...

I did however stumble upon this most wonderful site, which just might be the ultimate source for your specific question on quantity estimate: Home of Wool.  They offer wool stuffing and batting, as well as custom wool cushions and mattresses, not to mention their shop bursting with cool stuff made of wool and other natural fibers.  I am certain these friendly people would very happily assist you on your quantity question.

Foam Order is another source for organic wool batting, including one manufactured with cotton backing.  (They do not offer wool stuffing, though. They do discuss how to fold wool batting for the stuffing of a pillow, however, which sounds intriguing...)

Lastly, I will just throw in this little tutorial on how to stuff a cushion with wool.

And of course as you pointed out, there are wool battings available at Joann.

Happy reupholstering!