The answer is no in almost all cases. The number reported as gamut size is the volume of a three-dimensional solid. The problem you are describing is like trying to calculate how much an invisible container can hold given the coordinates of a few points that are inside or on the edge of the container. You might be able to do this if you had a lot
of points that were carefully picked (this is how profiles are made) or knew they were a random sampling inside and outside the gamut (see this post
for how that might work). Also, if you knew something about the shape of the container and knew where the points fell relative to the container. For example, you could calculate the volume if you knew the container was a cube and you had the coordinates of two vertices.
But with just a handful of colors and no more information, you can't really determine the volume.