It seems that you do the right thing. If the segmentation looks correct in slice and 3D views then the computed volume will be correct, too.
If the segmentation is not accurate then start it again. If you are not yet confident in using the Segment Editor then I would suggest to complete all available segmentation tutorials. You may also call in the weekly hangouts (see posts in community category about dates×) or can attend a project week to learn from Slicer users and developers in person.