The center of gravity of a standing human body is roughly about an inch below navel:
https://www.sciencedirect.com/topics/physics-and-astronomy/center-of-gravity

For a seated person the CoG is shifted forwards and up of navel, total distance and angle will very according to the specifics of the posture:
https://www.researchgate.net/figure/Center-of-gravity-of-a-person-in-sitting-posture-Body-segmental-Lengths-are-reported-in_fig2_256203106

The antrophomorphic test dummies mimic the proportions, weight distribution and kinematics of a human body as close as possible, for obvious reasons: to accurately replicate the response of a human body to forces, the ATD must resemble it accurately, not just as a whole, but down to its individual part also.

Example information of THOR ADT: [Anthropomorphic Test Devices; THOR 50th Percentile Adult Male Test Dummy; Incorporation by Reference][1]

If you wish to build a more accurate model, here is an example of a further break-down of proportional masses of average human body parts ([Source][2]):

|Segment|Male|Female|
|--|--|--|
|Head|8.26%|8.20%|
|Whole Trunk|55.1%|53.2%|
|Thorax|20.1%|17.0%|
|Abdomen|13.1%|12.2%|
|Pelvis|13.7%|16.0%|
|Total Arm|5.70%|4.97%|
|Upper Arm|3.25%|2.90%|
|Forearm|1.87%|1.57%|
|Hand|0.65%|0.50%|
|Forearm & Hand|2.52%|2.07%|
|Total Leg|16.7%|18.4%|
|Thigh|10.5%|11.8%|
|Leg|4.75%|5.35%|
|Foot|1.43%|1.33%|
|Leg & Foot|6.18%|6.68%|


  [1]: https://www.federalregister.gov/documents/2023/09/07/2023-19008/anthropomorphic-test-devices-thor-50th-percentile-adult-male-test-dummy-incorporation-by-reference
  [2]: https://robslink.com/SAS/democd79/body_part_weights.htm