Photoshop USM and CMYK conversion

[zxiong@xxxxxxxxxxxxxxxxxxxxxx]

Hi,

I need to convert some 40 in images (at 300 dpi) to CMYK.  I scanned myself
with unsharp masking turned off because of excessive stitching and retouching.
I wonder if there are any rules of thumb using USM in photoshop for images
of given sizes.  I found an amount of 150 % with a radius of 3 pixels at a
threshhold of 5 to 10 seemed to be fine.  I'd like to make sure if this is
the best combination before I would regret. Lowering the threshhold would mess
up the sky.  Or would it be better to sharpen different parts of the images
separately?   Any help would be greatly appreciated.

Zonghou Xiong