>One small point is that I believe 75 dpi is the screen >resolution on Apple monitors and PC monitors are 96 dpi.
I don't think that makes sense. Screen resolution is a function of the monitor, not the computer. My Dell U2711 has a pixel pitch of 233 microns (whether or not connected to a computer). That's because it was manufactured with 2560 x 1440 pixels across an area with a diagonal of 27". When I use the monitor in its native mode (as I usually do, WQHD, or 2560 x 1440) and view a photograph at 100%, I am getting 109 pixels across every inch of screen.
Going back to the OP's topic (a great one!), when I view his images they are possibly of different physical size than when someone else views them on their computer. That means we are each seeing a different magnification as compared with real life, which then means we are looking at different data to interpret whether some motion blur is significant. Jon, in a later post in this thread, has it exactly right in my view: we need to consider the amount of blur per printed inch when viewing the print at a usual distance.