Don't know of a scientific formula; however, calves tend to gain between 1.5 and 3.0 lbs a day, depending on forage and feed consumption. This is NOT related to "force feeding" of feedlot animals that you are trying to finish quickly and sell them soon. Take their birthweight plus say a "middle" gain figure (e.g., 2.0 lbs/day)and multiply the gain figure by the number of days since birth. Also, if you put an animal in a chute (or otherwise a calf in halter) take a "Cattle Weight Tape" and measure it around the chest in a line behind the front legs. These tapes we have found to be 90% or better accurate. Tapes available at larger farm and ranch stores and mail-order livestock supply places...cost around $5. or less.