I'm not aware of a modifier key that allows for fine-tuning on the fade tool. Zoom level affects this. Also, keep in mind what you see is not what you get - the fades can graphically appear longer than they are if you are zoomed out. To get a fine-tune while zoomed out click-drag the number field in the inspector instead of using the fade tool. It's blank if there is no fade yet, but you can drag and apply very small amounts. If you are not sure where to click-drag, use the fade tool and look at the inspector to see the #'s change. You can select several regions and put the same fade all at once.
While looking at the inspector, you might see the words "Eq. pwr" or something like that. IIRC when you set-up a cross fade with certain settings, and then try to change it to just a fade out it won't work. You need to change it back to 'fade out' or whatever.