I'm not a fan of the seatpost-mounted racks, especially when loaded that heavily on a long tour. Seems like a long cantilever that puts quite a bit of extra load on the post so make sure the post is strong enough. I've also seen a number of these racks swing sideways. There are racks that attach to the quick release axle at the bottom and to either the brake bridge hole or to p-clamps around the seat stays at the top (Old Man Mountain is one maker of these). I'd recommend using one of these instead of a seatpost rack.