Last fall we freeze branded our heifers for the first time. They weighed about 650lbs. We used liquid nitrogen rather than dry ice. Nitrogen gets the irons colder than the dry ice. We left the irons on for 22 seconds. That wasn't quite long enough. Next year we are going to try 27 or 28 seconds per iron.
You will want someone with you running a stop watch. After you shave the branding location, you need to wet the area down with alcohol, we used a squirt bottle, it worked OK. This helps transfer the tempature to the hide. You might want to get a pair of welding gloves, they are a thicker than regular gloves, when dealing with something around 145 degrees below zero you need to be very careful.
good luck.