
from where will it be 90mm (nominally) to the film plane when focussed at infinity?
The correct answer is the rear nodal point or principal plane. For the equation 1/Dsubject + 1/Dimage = 1/f to work, the distance to the subject is measured to the front nodal point and the distance to the image is measured to the rear nodal point. On quasisymmetrical lenses the nodal points tend to be close together and near the aperture. The positions can be far from the aperture for telephotos and wideangle lenses for SLRs.
As jford realized, for most photographers the flange focal distance (FFD) is the most useful number  this the distance from the image to back of the shutter when the lens is focused on infinity. Most LF manufacturers provide this value. Some provide the positions of the nodal points. If the data isn't available, there are ways of determining these values.