To understand how the volume is calculated, click on the toolbar button that has a green hash on it. This will indicate the area in which the wall will be turned. This hash mark looks at adjacent rows and identifies the part where the two rows overlap. The green hash identifies the trapezoid from the lower overlap to the upper overlap. Clicking the toolbar icon that looks like a glue bottle will show you how wide those overlaps are.
The volume is calculated by row and it is based on the average inside diameter of the row based on the lowest point of the inside hash mark and the highest point of that hash so it takes the slope of the wall profile into consideration.. It doesn’t matter whether profile is contracting or expanding. This average and the actual height of the ring provides the volume for that row. This volume is added to the value of all of the rows, ending at 1-1/2” from the top where the top cap is likely to be. In my opinion, the volume calculation is very accurate. In fact, it is far more accurate than the thumbnail estimate of one pound of human ashes equals one cubic inch of volume. That calculation can be off by 10-20%.