The mathematical model is based on a cone. The cone, or log, is easily changed with respect to taper, top diameter and length. The yield is based on the nominal dimension of the boards and length with or without wane.
The log volume is calculated in two ways, true volume or top cylinder volume. The true volume is the volume of the cone. The top cylinder volume is based on top diameter and length.
The calculation will give theoretical values. In practice the roundness of a log is not a perfect circle, the logs are not perfectly straight and will have sweep and crook. The measurement accuracy of logs and boards can also affect the results. Other factors include errors in feeding etc.
Sawmills using SDM will soon notice how much these imperfections/errors affect the actual yeild compared to the theoretical yeild. It is therefore possible to adjust for these mill-specific deviations in SDM by specifying a compensation factor.