Once segmental center of mass positions are known, these can be used with the segment mass percents to determine a "torque" about an axis. This is really a (mass)*(length) calculation.
If the torques created by each segment are summed the result is equal to the torque of the whole body mass acting at the center of mass location. This idea allows determining the unknown whole body CM location.
In your spreadsheet, add a column with the segement mass percents. Then write formulas for each segment's torque in each direction. Finally, sum the torques and divide by total body mass (100%).