Have you previously gotten a Saddlesack to work with an R10 rack? I couldn't.
That rack seems more suited to traditional bags, like a Caradice, that are improved by swinging them away from the seatpost and out from under the saddle. Rivendell (sackville) bags are kind of hung from the center of mass, and only reall work when allowed to hang level, under the saddle, and strapped to the seatposti front.
If I'm wrong, I'd love to see photos of somebody successfully mating the two. Otherwise, just something to consider before buying.