[Chimera-users] shape factor

Hello, We are using Chimera to classify viruses. We use pdb files from viper. We want a way to obtain exactly shape factor and/or compare with an icosahedron. We have problem do it. We open pdb file, draw icosahedron using "hkcage" (all models actives) and try to compare that two figures using "fit in map", or similar but doesn't work. What is the best and exactly way to do it? Thank you.