-
1. Re: view at actual size in CS4 on iMac is 75%
You need to set your screen resolution in the Units & Rulers tab of the preferences to match your display's ppi.
http://en.wikipedia.org/wiki/Pixel_density
http://en.wikipedia.org/wiki/List_of_displays_by_pixel_density
-
2. Re: view at actual size in CS4 on iMac is 75%
Well theres Actual size and Print size.
Actual size will display images as 1px for each px on your screen resolution. So if you made a document 1280 px wide and viewed it at Actual size it should take exactly half of the screen.
Print size on the other hand has to be calculated via the number of pixels set in your screen resolution divided by a direct physical measurement of your screen. Then set that number (which will probably be closer to 100 than 72) in Photoshops Preferences > Units and Rulers > Screen Resolution.