You should measure a related variable like shoot basal radii and use allometric scaling on a robust sample. The principles are drawn in Brian Enquist https://www.researchgate.net/profile/Brian_Enquist/publication/11051612_Universal_Scaling_in_Tree_and_Vascular_Plant_Allometry_Toward_a_General_Quantitative_Theory_Linking_Plant_Form_and_Function_from_Cells_to_Ecosystems/links/09e4150c6bb5e827d2000000.pdf and subsequent papers. On big trees trink radius is the main variator.
Article Universal Scaling in Tree and Vascular Plant Allometry: Towa...