With Photo CD, it is applied after the scan. USM is applied in Photoshop in the filter menu under sharpen. The percentages for each of the 3 parameters will vary from image to image and also from output device to output device. A sharp, crisp image can take more USM, while a soft grainy image may actually look worse when USM is applied. An image heading to four color separations can take more USM than one heading to a continuous tone film recorder.
Larger files can take more than smaller files.
Be aware that an image may appear oversharpened on the screen but look fine when output. Experiment with the percentage, pixel radius and threshold values until you produce satisfactory results. Save your file in the format of your choice. Now your image is color corrected and sharp and ready to output.