Have a look in the Help files in PS Barry, I don't use PS, but generally speaking you need different USM settings depending on whether your output is to a monitor or to print.
The other thing is you don't want to be looking at a 'one shot' USM setting for all images, as images will vary in the amount of sharpening they need anyway, but you ideally need very low USM settings and apply USM in small increments, which allows you fine control over any halos or sharpening artifacts appearing, in which case you can just undo the last USM step or two if need be.
I usually end up applying USM in several increments for best results. It might sound like a bit of a faff, but it isn't in practice. Obviously you should do your sharpening at the final output size of the image.
