Genetic size-scaling rules (Genetic size-scaling:  St C. S. Taylor. Genetic size-scaling rules in animal growth. Anim. Prod. 30:161-165, 1980.):

1. Treat all age and time variables as directly proportional to A^0.27, where A is mature (adult) body weight.
2. At every age standardized as in (1), treat all cumulated inputs and outputs as directly proportional to A.

Metabolic age, represented by q, is calculated as age from 3.5 days after conception divided by A^0.27.  For example, a 100-day-old Dorset ewe lamb with a potential mature size of 70 kg and a gestation length of 146 days would have a metabolic age of (100 + 146 - 3.5)/70^0.27 or 242.5/3.15 or 77 metabolic days.

An example with Dorset and Suffolk rams (A. M. Oberbauer, A. M. Arnold, and M. L. Thonney. Genetically size-scaled growth and composition of Dorset and Suffolk rams. Anim. Prod. 59:223-234, 1994.)