yes swim thinks  the weight of the green layer can assure you % of mescaline  not the whole cutting swim has found out that some san pedro can be very big like 3 metre tall even more, but they are thinner then others that even are smaller , and when i see there interior  some of these Thin ones  the green layer is the same quantaty as the white part i think this is because the san pedro have not had much water and others have much more water in them so i think that the weight of the whole cacti
not always can assure you the mescaline that the san pedro haves but the green part does 
om namah shivaya