A much more detailed look at precision manipulation workspace is now available through our paper in IEEE Transactions on Biomedical Engineering (TBME). Below is an example figure from the paper. This figure shows typical finger positions relative to the object in both two- and three-fingered manipulation of a small object in the fingertips.
The thumb position appears to stay fairly constant, while the position of the other fingers is significantly different depending on whether two or three fingers are used. This data could be used, for example, to anticipate natural finger placement when designing devices which are held in the fingertips during use. Please see the paper for more details.