Page 57 of Tree Root Damage to Buildings describes the relationship between gravimetric and volumetric moisture content, and provides the correct formulae for converting between the two.

However, at page 266 and figure 16.11 it provides a worked example. In this example, the gravimetric moisture content of the desiccated and control boreholes are first converted to volumetric moisure content, and the difference between the volumetric values is then used to calculate the layer deficit. This fails to take account of the change in volume which is occurring, and thus gives an erroneous result. The correct formula to use, and the correct method, are described in paragraph 5 on page 6 and Table 2 of BRE Digest 412.