I have just had a look at the site and this is an extract 'A formula was devised to compare the weight-to-length ratio, which is known as ‘The Jackson Ratio’. However, this ratio can only be applied to the Testudo graeca (Mediterranean Spur-thigh tortoise) and Testudo hermanni (Hermanns tortoise) and CANNOT BE USED FOR ANY OTHER SPECIES due to the different body shapes.' So yes this will be useless for any other species.