Ok, I now remember that prior to the STAR Tools functions, instead of using the MeasureContainerVolume2 function (likely because I ran into similar issues), my approach was to use the step return to get the liquid level found, calculate the height of the liquid in the container and then feed that into the HSLDevComputeContainerVolume function. See this link for an example method. My apologies, this was written before we had a proper variable naming convention.
The STAR Tools function is much more straightforward though.
Generally speaking, the volume estimates are typically going to underestimate the volume for a variety of reasons (meniscus of the liquid, labware definition, etc.). For better volume estimates, avoid using a “v-cone base segment” in your container definition as the calculation for geometry is not correct and will underestimate the volume. Replace it with a “v-cone” with a smaller lower internal diameter.