The Zipp 303 or 404 Firecrest tubular wheels paired with the Avid Shorty Ultimate brakesets are universally considered the best wheel/brake combination you can run on your cyclocross bike. The problem is, they are not compatible. The rims are far too wide for the brake shoes that come stock on the Avid Ultimates.
Enter the 25.5mm brake shoe. This shoe has a post length of 25.5mm, which is shorter than the stock post. It makes running the wider Firecrest wheels with the Ultimate brakes a seamless operation. Simply swap out the stock shoes for these, adjust up your brakes and you will experience cyclocross bliss. The pads that come with these shoes are SwissStop RacePro Yellow carbon-ready pads. These can be used as front or rear and are sold in pairs (2 individual brake shoes). Make sure you buy two of these if you are setting up one complete bike.
These pads are sold in pairs and are enough for one brake. You will need to purchase two in order to set up an entire bike on both front and rear. These are not front and rear specific they will work on either.