Calculating brain-part volume percentage?
Short answer
Magnetic resonance imaging can be used to calculate volumes of brain areas. 

Background
A state-of-the-art method to determine volumes in brain tissue is by making use of magnetic resonance imaging (MRI). MRI segments the brain into 3D pixels, called voxels. Voxel is derived semantically from the fusion of pixel and volume. Hence, each voxel has a certain volume and thus the number of voxels inside a certain brain area (e.g. the hippocampus) can be calculated yielding your desired volume measures (Fig. 1). 

Of course you cannot do MRI in your attic, but you can search the internet for the right articles. Two leads are cited below.


Fig. 1. Brain divided into voxels by MRI. source: Everything Zoomer

References
- Andica et al., J Neuroradiol (in press)
- Aquartz et al., Eur Arch Psychiatry Clin Neurosci (2001);251(6): 255-61