the measure of the seam for "proportion" is to have a relation between your upper body and the lower part, it is to know how long your leg in proportion to the body,

an example I have 80cm from the bottom of my body and 1 meter from the
"superior part", total 1.80m. within the body proportion is just that,
but it is obvious that in order to have the actual measurements you measure the tibia from the inside of the leg to the lateral bone of the ankle, and to measure the femur you measure by the side looking with the finger of the head of the femur on the side of the hip and until the intersection of the knee.

https://imgur.com/a/PEVQjnw